Heat 2 Tbsp oil and 1 Tbsp. butter in a large skillet over medium-high until butter is melted. Cook mushrooms, tossing occasionally, until browned and tender, about 5 minutes. Season with salt, then transfer to a plate.
Combine 1 Tbsp oil and remaining 1 Tbsp butter in same skillet. Season steak generously with salt and pepper. Cook until well browned, 3–4 minutes per side for medium-rare, depending on thickness. Transfer to another plate; wipe out skillet.
Reduce heat to medium and pour remaining 1 Tbsp oil into pan. Cook shallots, stirring often, until softened, about 2 minutes. Add garlic; cook, stirring, until softened, about 1 minute. Add tomato paste and cook, stirring constantly, until slightly darkened in color, about 2 minutes. Add flour and cook, stirring often to coat shallots and garlic, until toasty smelling, about 3 minutes. Add wine; cook, scraping up browned bits, until evaporated, about 1 minute. Add broth, fish sauce, and any liquid from mushrooms (leave mushrooms behind). Bring to a boil over high heat, then reduce heat; simmer, stirring occasionally, until sauce coats a spoon, 12–15 minutes.
Slice steak against the grain. Place sour cream in a small heatproof bowl and gradually whisk in 1 c sauce. Stir back into sauce in skillet. Taste and season with more salt or fish sauce if needed. Stir in mushrooms. Serve sauce over noodles, topped with steak and scallions.
Tags: beef, bonappetit, dinner, egg-noodle, main, mushroom, pasta, russian, stew
