Adapted from Bon Appétit
3 tablespoons extra-virgin olive oil, divided
12 ounces assorted mushrooms, torn or cut into large pieces
Kosher salt, freshly ground pepper
4 tablespoons (1/2 stick) unsalted margarine, divided
1-1/2-pound hanger steak, trimmed, pounded to 1/2" thickness (you can also use skirt steak)
Coarsely cracked black pepper
3 garlic cloves, lightly crushed
1 (6-inch) sprig rosemary
1 cup dry red wine
3/4 cup low-salt chicken stock
2 tablespoons chopped fresh tarragon
Heat 2 tablespoons oil in a large heavy skillet over medium-high heat. Add mushrooms; cook, stirring occasionally, until soft and golden, about 7 minutes. Season to taste with salt and pepper. Transfer to a bowl; set aside.
Melt 1 tablespoon margarine with remaining 1-tablespoon oil in same skillet over medium heat. Season steak with salt and cracked pepper. Add steak, garlic, and rosemary to skillet. Cook about 3 minutes per side for medium-rare. Transfer to a cutting board. Let rest while preparing sauce.
Discard garlic and rosemary from skillet. Pour off all but 1 tablespoon fat. Add wine; cook, stirring up bits, until reduced to 3/4 cup, about 3 minutes. Strain; return liquid to skillet. Stir in stock; bring to a boil. Simmer until reduced to 1/2 cup, about 5 minutes. Remove from heat; Whisk in 3 tablespoons margarine. Stir in mushrooms and 1 tablespoon tarragon. Season with salt and pepper.
Spoon mushroom mixture onto plates. Thinly slice steak; serve over mushrooms. Garnish with remaining 1 tablespoon tarragon.