Step 1

Heat oil in a large skillet over medium-high heat. Season steak with salt and pepper. Cook, in 2 batches, without turning, until golden brown on one side and mostly cooked through. Transfer to a plate.

Step 2

Add butter and onion to skillet. Season with salt and pepper. Cook, stirring occasionally, until onion begins to soften, 1 to 2 minutes. Add mushrooms and cook, stirring occasionally, until vegetables are tender, 3 to 4 minutes. Sprinkle flour over vegetables and cook, stirring, 1 minute. Stir in stock and vermouth. Cook, stirring occasionally, until thickened, 6 to 8 minutes.