Grilled Flat Iron Steak Recipe with Garlic Herb Butter

Our easy recipe for a Grilled Flat Iron Steak pairs perfectly grilled, tender flat iron steaks with a savory shiitake mushroom topper. The mushrooms add deep, earthy umami and a bright finish from a splash of raspberry red wine vinegar that elevates the whole dish.

Grilled Flat Iron Steak

Few things beat a well-cooked steak. A flat iron steak is flavorful, tender, and responds beautifully to high-heat grilling. Keep the seasoning simple—salt and pepper—to let the beef shine, then finish each steak with a quick sauté of shiitake mushrooms for a memorable topping.

This mushroom topper is surprisingly addictive. The combination of butter and shiitakes, brightened with a touch of raspberry red wine vinegar, complements the meaty richness without overpowering it. It’s simple to prepare and worth stocking the mushrooms for the next time you fire up the grill.

Why You’ll Love This Dish

Robust Flavor: The flat iron’s beefy profile pairs perfectly with the umami-rich shiitake mushrooms.
Easy Grilling: Grilling a steak is straightforward and delivers restaurant-quality results at home.
Minimal Ingredients: Few, high-quality ingredients deliver maximum taste.
Flexible Serving: Serve simply for a low-carb plate or add sides like potatoes or grilled vegetables.
Quick to Make: From start to finish this recipe comes together quickly—ideal for weeknight grilling or entertaining.

Get 75+ favorite Traeger recipes in one place for more grilling inspiration.

Recipe Shopping List

Here are the items you’ll need. For exact amounts, see the recipe card below.

Flat iron steaks
Salt
Pepper
Fresh shiitake mushrooms
Butter
Raspberry red wine vinegar

Serve with a smoked baked potato or a simple green vegetable for a complete meal.

How To Make This Recipe

Below is a straightforward overview of the cooking process. Follow the full recipe steps in the card at the end for timing and temperatures.

Preheat the grill: Heat to medium-high, about 400–425°F (205–220°C).
Season the steaks: Generously season both sides with salt and freshly ground black pepper.
Grill the steaks: Place steaks on the grates and cook until they reach your desired doneness, flipping once.
Make the mushroom topper: While the steaks cook, sauté chopped shiitake mushrooms in butter, season lightly with salt, and finish with a splash of raspberry red wine vinegar.
Rest and serve: Let the steaks rest a few minutes after grilling, then top with the warm mushrooms and serve.

Pair with one-pot parmesan noodles with asparagus or a fresh Caesar salad for a fuller meal.

Recipe FAQ

How long should I grill the steaks?

For medium-rare, aim for an internal temperature of 130°F (54°C). On average, this is about 6–7 minutes per side depending on steak thickness and grill heat. Use an instant-read thermometer for best results.

Can I prepare the mushroom topper in advance?

Yes. Sauté the mushrooms ahead of time, then reheat gently just before serving so they stay tender and flavorful.

Are there substitutions for shiitake mushrooms?

Button or cremini mushrooms work well as substitutes, though the flavor will be slightly different—still delicious, just less pronounced in umami.

Serve This With

Traeger smashed potatoes or roasted potatoes
Grilled asparagus or green beans
Strawberry salad with red wine vinaigrette or a simple mixed greens salad
Garlic mashed potatoes

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Reverse-seared flat iron and hanger steaks
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Explore other steak recipes for inspiration and different cooking techniques like reverse searing, smoking, or using a griddle.

Yield: 4 people

Grilled Flat Iron Steak with Shiitake Mushrooms

These steaks are ideal for sharing. Cooked to the preferred doneness and topped with savory shiitake mushrooms, they’re sure to be a repeat favorite.

Prep Time: 15 minutes

Cook Time: 45 minutes

Total Time: 1 hour

Ingredients

4 (8-ounce) flat iron steaks
2 tablespoons salt
1 teaspoon freshly ground black pepper

Mushroom Topper

½ pound fresh shiitake mushrooms, chopped
¼ teaspoon salt
2 tablespoons butter
1 teaspoon raspberry red wine vinegar

Instructions

Preheat your grill to 400–425°F (205–220°C).
Season the steaks evenly with salt and pepper.
Place the steaks on the grill grates and cook, flipping once, until they reach your desired internal temperature.
While the steaks cook, prepare the mushroom topper.
Chop the shiitake mushrooms into roughly ¼” pieces and heat a medium saucepan over medium heat.
Add the butter and stir in the mushrooms as it melts.
Season the mushrooms with salt and sauté until browned at the edges.
Stir in the raspberry red wine vinegar, turn off the heat, and keep warm for 1–2 minutes.
For medium-rare steaks, remove when internal temperature reaches 130°F (54°C). Let steaks rest 5–10 minutes.
Top each steak with the warm mushrooms and serve immediately.

Nutrition Information (per serving)