Flank Steak with Mushroom Sauce

⏲ ~18 min total · prep 4 · cook 14 · 4 servings

By the KFI Test Kitchen · Tested & approved by Jason Lieu, KFI recipe editor

Published Sep 15, 2026 · Updated Sep 15, 2026

Ingredients

• 1 1/2 lb flank steak
• 8 oz cremini mushrooms, sliced 1/4 inch thick
• 1 shallot, minced
• 2 cloves garlic, minced
• 3 tbsp butter, divided
• 3/4 cup beef broth
• 1 tbsp tamari
• 1 tsp fresh thyme leaves
• 1 tsp Dijon mustard
• kosher salt and black pepper

Instructions

1. Season the steak on both sides with salt and pepper and let it sit on the counter 20 minutes while a heavy 12-inch skillet heats over high until a drop of water skitters.
2. Sear the steak 4-5 minutes per side for medium-rare (125-128°F internal), pressing it flat with a spatula so the whole surface crusts.
3. Transfer to a cutting board and tent loosely with foil; rest 8 minutes while you make the sauce.
4. Drop the heat to medium-high, add 1 tbsp butter and the mushrooms, and cook 5-6 minutes, stirring rarely, until browned and their liquid has fully evaporated.
5. Add the shallot and garlic; cook 1 minute until fragrant. Pour in the broth and tamari, scrape up the browned bits, and simmer 4-5 minutes until reduced by half.
6. Pull the pan off the heat and swirl in the remaining 2 tbsp cold butter and the Dijon until the sauce turns glossy; stir in the thyme.
7. Slice the steak thin, across the grain, and serve with the mushroom sauce spooned over.

Why this recipe works

• Across the grain Flank steak runs long fibers the whole length of the cut. Thin slicing across them shortens every strand, which is where the tenderness actually comes from.
• Cold butter finish Two tablespoons of cold butter swirled in off the heat turn the pan drippings into a glossy sauce. No flour, no roux, no separate steps.
• Dry-seared mushrooms Creminis left alone in a single layer brown instead of steaming. Their liquid evaporates fully, which is what lets the broth reduce into something concentrated.

Tips for making this recipe well

Twenty minutes on the counter

Cold steak sears the outside long before the center cooperates. Sitting out takes the chill off so the 125 to 128°F window lands evenly.

Press it flat

A spatula pressed on the steak during the sear keeps the whole surface against the pan. Full contact means full crust.

Wait for the dry sizzle

The mushrooms are ready for the shallots when their liquid is gone and the pan squeaks. Rush this and the sauce starts watery.

Off the heat for the butter

Swirl the cold butter in after the burner is off. Boiled butter sauce turns loose and greasy; swirled, it clings.

Rest the full 8 minutes

The foil tent holds enough heat while the juices pull back into the meat. Cutting early spills them onto the board instead.

Serving suggestions

Slice the steak thin, across the grain, fan it on warm plates, and spoon the mushroom sauce over the top. Against the grain is not a garnish-level detail with flank; the fibers run the length of the cut, and cutting across them is the difference between tenderness and a workout. At 4.0 net carbs a serving, the sauce can be generous. Rest those 8 minutes while the sauce comes together, and serve the moment the butter goes glossy. This plate waits for no one.

Storage and meal prep

Refrigerate the steak and sauce together, covered, for 3 to 4 days. Sliced leftovers reheat best in a short turn in a skillet over low, tucked into the sauce so the meat warms without cooking further. Sliced thin and stored flat, the steak also freezes for 2 to 3 months, though it will be a shade less tender after thawing; the sauce comes back with a slow warm-up and a stir. Cold slices straight from the fridge are honestly good too, laid over greens.

Variations worth trying

• Skirt or hanger Both cuts take the same hot sear and the same 125 to 128°F pull. Slice across the grain the same way.
• Creamy version Stir a quarter cup of heavy cream into the sauce with the broth. It turns the pan sauce silky and slightly paler.
• Coconut aminos Swap the tamari one for one. The sauce runs a touch sweeter and softer around the edges.
• Thyme out Rosemary or a small sprig of sage replaces the thyme. Add either with the broth so it mellows.

Frequently asked questions

What temperature should flank steak be?

This recipe pulls it at 125 to 128°F for medium-rare, with carryover during the 8-minute rest. If you prefer it more done, 145°F with a rest is the usual floor for whole cuts.

Why are my mushrooms gray and wet?

The pan was crowded or the mushrooms were stirred too often. A single layer, left alone, releases its liquid, evaporates it, and then browns.

Can I marinate the steak first?

Flank takes a marinade well, but pat it completely dry before searing or the crust never forms. Skip anything sugary, which burns at searing heat.

My sauce broke and looked oily. What happened?

The cold butter was added over active heat and boiled. Pull the pan off the burner first, then swirl the butter and Dijon until the sauce turns glossy.

How thin should I slice it?

Quarter-inch slices, cut across the grain, are the target. A long, sharp knife and a slight angle make the pieces wider and more tender on the plate.

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