Marinated Flat Iron Steak

⏲ ~12 min total · prep 4 · cook 8 · 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 flat iron steak
• 3 tbsp olive oil
• 2 tbsp coconut aminos
• 1 tbsp red wine vinegar
• 2 cloves garlic, minced
• 1 tsp Dijon mustard
• 1/2 tsp smoked paprika
• 1/2 tsp black pepper
• 1/2 tsp fine salt

Instructions

1. Whisk the olive oil, coconut aminos, vinegar, garlic, Dijon, smoked paprika, and pepper in a baking dish, then turn the steak in the marinade to coat every surface.
2. Marinate at room temperature 40 minutes, flipping once halfway — flat iron is tender enough that a short soak does the job.
3. Pull the steak from the marinade, letting the excess drip off, and reserve the dish with its liquid.
4. Heat a cast-iron skillet over high until it just begins to smoke; add the steak and sear undisturbed 4-5 minutes until a dark mahogany crust forms, then flip and sear 3-4 minutes more for 128-130°F at the center, medium-rare.
5. Move the steak to a cutting board and rest 8 minutes — the carryover takes it to a rosy 135°F.
6. Meanwhile pour the reserved marinade into the hot skillet and boil hard 2 minutes; it is safe because the raw steak has been out, and it reduces to a syrupy glaze.
7. Slice the steak against the grain into thin strips and spoon the warm glaze over.

Why this recipe works

• Flat iron earns its cut Flat iron is the second-tenderest muscle on the animal, so a forty-minute marinade and a hard sear are all it asks. A longer soak does nothing for a cut this tender.
• Coconut aminos marinade It brings the soy-sauce depth without the soy, and at 7 net carbs the whole plate stays keto. The Dijon in the mix emulsifies the oil so every surface gets coated.
• The marinade becomes the sauce The reserved liquid boils hard in the same skillet for two minutes and reduces to a syrupy glaze. Nothing made separately, nothing wasted.

Tips for making this recipe well

Pat off the excess marinade

Wet meat steams before it sears, and the crust suffers. Let the steak drip, then lay it into the smoking-hot pan undisturbed.

Wait for the smoke

Cast iron over high heat until it begins to smoke is the cue. Underneath that point, the steak sits in the pan longer and cooks gray instead of mahogany.

Trust the clock, then the thermometer

Four to five minutes undisturbed, three to four after the flip, lands the center at 128 to 130°F. The eight-minute rest carries it to a rosy 135°F.

Boil the marinade the full two minutes

The raw steak has been out of it, and a hard boil makes the reserved liquid safe and thick. It should coat the spoon like warm syrup when it comes off the heat.

Slice against the grain

Flat iron has a defined grain, and cutting across it shortens every fiber. Thin strips, warm glaze over the top, done.

Serving suggestions

Slice against the grain into thin strips, fan them on a cutting board or platter, and spoon the reduced glaze over while it is still warm and pourable. Sixty minutes includes the forty-minute marinade, so the active work is one sear and one boil. Each serving carries 7 net carbs. This is a steak that eats like a weeknight and plates like company; a pile of arugula with lemon underneath catches the extra glaze and asks nothing back.

Storage and meal prep

Cooked steak keeps 3 to 4 days in the fridge, tightly covered, and is best eaten cold or barely warmed over medium-low heat, because a hot reheat pushes the medium-rare center well past rosy. The glaze keeps alongside in its own container and thickens when cold; a few seconds of heat loosens it. Freezing works for 2 to 3 months, thawed overnight in the fridge, though the seared crust softens and the interior loses some of its rosiness either way.

Variations worth trying

• Grill instead High direct heat on a hot grill does the same job as cast iron. Pull the steak at 128 to 130°F and rest it the full eight minutes regardless.
• Rosemary in the marinade A chopped sprig with the garlic shifts the glaze toward steakhouse territory. It plays well with the coconut aminos.
• Faster marinade Twenty minutes works in a pinch for this cut. Flat iron is tender enough that the marinade is mostly about surface flavor anyway.
• Chile flakes A quarter teaspoon in the marinade adds a slow warmth to the glaze. The reduction concentrates it, so start small.

Frequently asked questions

Why only forty minutes of marinating?

Flat iron is already tender, so the marinade is seasoning, not tenderizing. Past an hour or two, the vinegar starts to change the surface texture for no gain.

Can I marinate overnight?

You can, and it will not hurt the flavor, but it does nothing the forty minutes did not. Pat the surface dry well before it hits the pan either way.

Is it safe to use the marinade as a sauce?

Here, yes, because the method has the raw steak out of the dish and boils the liquid hard for two full minutes. That reduction makes it both safe and thick enough to glaze with.

What if I want it more done than medium-rare?

Add two to three minutes on the second side and check with a thermometer. Pull it about five degrees shy of your target, since the eight-minute rest carries it further.

No cast iron skillet?

Any heavy, oven-safe skillet that can take high heat works. Stainless is fine; a thin nonstick pan will not build the same crust at these temperatures.

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