Blackstone Flank Steak Over Garlic Mashed Potatoes with Mustard Worcestershire Onion Sauce

Make juicy Blackstone flank steak over creamy garlic mashed potatoes with a homemade mustard-Worcestershire onion sauce. An easy, flavor-packed Blackstone dinner made entirely on the griddle.

Marinated Flank Steak Over Creamy Garlic Mashed Potatoes

Prep Time: 20 minutes

Marinate Time: 2–8 hours

Cook Time: 35–40 minutes

Total Time: About 1 hour + marinating

Servings: 4–6

Difficulty: Moderate

Ingredients

Flank Steak

1½–2 lb flank steak

Oil for the Blackstone, as needed

Kosher salt, as needed

Freshly cracked black pepper, as needed

Mustard-Worcestershire Steak Marinade

¼ cup Worcestershire sauce

3 tablespoons Dijon mustard

2 tablespoons apple cider vinegar

2 tablespoons olive oil

3 garlic cloves, minced

1 tablespoon brown sugar

1 teaspoon freshly cracked black pepper

½ teaspoon kosher salt

Garlic Cream Cheese Mashed Potatoes

2 lbs Yukon Gold or Russet potatoes, peeled and cut into chunks

4 oz cream cheese, softened

4–5 garlic cloves, minced

½ cup heavy whipping cream, warmed

4 tablespoons butter, divided

1–1½ teaspoons kosher salt, or to taste

Freshly cracked black pepper

Fresh parsley, chopped, optional

Mustard-Worcestershire Onion Sauce

1 large yellow onion, thinly sliced

2 tablespoons butter

1 tablespoon Dijon mustard

2 tablespoons Worcestershire sauce

1 tablespoon apple cider vinegar

½ cup beef broth

1 teaspoon brown sugar

½ teaspoon garlic powder

Freshly cracked black pepper

1 tablespoon cold butter

Directions

1. Marinate the Flank Steak

Whisk together the Worcestershire sauce, Dijon mustard, apple cider vinegar, olive oil, minced garlic, brown sugar, black pepper, and kosher salt.

Place the flank steak in a resealable bag or shallow container and pour the marinade over the steak. Make sure the entire steak is well coated.

Refrigerate for at least 2 hours or up to 8 hours.

Remove the steak from the refrigerator about 30 minutes before cooking.

2. Cook the Potatoes on the Blackstone

Place the peeled and chopped potatoes into a saucepan and cover with cold water. Season the water generously with kosher salt.

Place the saucepan directly on the Blackstone over medium-high heat.

Bring to a boil and cook for approximately 15–20 minutes, or until the potatoes are completely fork tender.

Drain thoroughly and return the potatoes to the warm saucepan.

3. Cook the Garlic

While the potatoes are cooking, melt 1 tablespoon of butter on a medium-low section of the Blackstone.

Add the minced garlic and cook for approximately 30–60 seconds, stirring frequently, just until fragrant.

Be careful not to brown or burn the garlic.

Add the cooked garlic and butter directly to the drained potatoes.

4. Make the Garlic Cream Cheese Mashed Potatoes

Add the softened cream cheese, remaining 3 tablespoons of butter, kosher salt, freshly cracked black pepper, and about half of the warmed heavy whipping cream to the potatoes.

Mash everything together until creamy.

Continue adding the remaining heavy whipping cream a little at a time until the potatoes reach your desired consistency.

Taste and adjust the salt and pepper.

Cover the saucepan and move it to the coolest section of the Blackstone to keep warm.

5. Cook the Onions

Heat another section of the Blackstone to medium heat.

Add 2 tablespoons of butter and the thinly sliced onions.

Cook for approximately 8–10 minutes, stirring occasionally, until the onions soften and develop some beautiful caramelization around the edges.

Move the onions to a cooler section of the griddle while you cook the steak.

6. Sear the Flank Steak

Heat another section of the Blackstone to medium-high or high heat.

Remove the flank steak from the marinade and allow the excess marinade to drip off.

Lightly oil the griddle if necessary and place the steak onto the hot Blackstone.

Let it cook undisturbed long enough to develop a good crust.

Cook for approximately 4–6 minutes per side, depending on the thickness of the steak and your preferred doneness.

For medium-rare, remove the steak at approximately 125–130°F and allow the carryover heat to finish cooking it while it rests.

Transfer the steak to a cutting board and let it rest for 8–10 minutes.

7. Make the Mustard-Worcestershire Onion Sauce

While the flank steak rests, bring the caramelized onions back into the area where you cooked the steak.

Add the Dijon mustard, Worcestershire sauce, apple cider vinegar, beef broth, brown sugar, garlic powder, and freshly cracked black pepper.

Stir everything together while using your spatula to work the sauce through the area where the steak cooked. This helps pick up all that concentrated flavor left behind on the Blackstone.

Allow the sauce to simmer and reduce until the onions are coated in a glossy golden mustard-Worcestershire sauce.

You don't want a thick brown gravy. Keep enough liquid in the sauce so it can run down through the steak and into those mashed potatoes.

Move the onions away from the heat and stir in 1 tablespoon of cold butter for a silky finish.

Taste and adjust the seasoning as needed.

8. Slice the Flank Steak

Look at the flank steak and identify the direction of the muscle fibers.

Using a sharp knife, slice the steak thinly against the grain.

I also like to hold the knife at a slight angle while slicing. This creates wider slices for a great presentation while shortening those muscle fibers for a more tender bite.

Build the Plate

Start with a generous bed of the garlic cream cheese mashed potatoes.

Fan the sliced Blackstone flank steak directly over the mashed potatoes.

Spoon the warm mustard-Worcestershire onion sauce generously over the steak, allowing those onions to fall naturally across the slices and some of that golden sauce to run down into the mashed potatoes.

Finish with freshly cracked black pepper and a little chopped fresh parsley.

Bright Spots Pro Tip 🔥

Don't clean the Blackstone after cooking the steak!

Once the flank steak comes off to rest, bring those onions right into the same cooking area before adding the beef broth, mustard, Worcestershire, and other sauce ingredients.

All those browned bits and concentrated steak juices left behind on the griddle become part of your sauce—and that's where some serious flavor is hiding.

And remember: always slice flank steak thinly against the grain. It makes a huge difference in tenderness.

Print Recipe Pin Recipe

Prep Time 20 minutes mins

Cook Time 35 minutes mins

Marinate Time 4 hours hrs

Servings: 4

Why You’ll Love This Blackstone Flank Steak Recipe

This is exactly the kind of meal I love making on the Blackstone because the entire griddle becomes part of the cook.

The potatoes are boiling in a saucepan on one section while the garlic and onions are cooking directly on the griddle. Then the flank steak gets that beautiful Blackstone sear before all those leftover flavors become the base for the mustard-Worcestershire onion sauce.

And when you put it all together—creamy garlic mashed potatoes, juicy sliced flank steak, caramelized onions, and that tangy savory sauce—you’ve got some serious comfort food.

Best of all, you never have to leave the griddle.

Until the Next One…

Sometimes the best meals aren’t complicated—they’re about taking a few great ingredients, building layers of flavor, and letting the Blackstone do its thing.

This one brings together juicy marinated flank steak, rich garlic cream cheese mashed potatoes, and a mustard-Worcestershire onion sauce that you’ll definitely want running down into those potatoes.

Give this one a try, make it your own, and most importantly—have some fun around the griddle.

Until the next one… we’ll see ya! 🔥