Skirt Steak with Shallot Sauce

Heat 1 tbsp butter and oil in a large cast-iron skillet over medium-high heat. Trim and season the steaks. Then cook them, two at a time, until medium rare, 3–4 minutes per side. Transfer steaks to a plate and cover with aluminum foil. Add shallots to skillet and cook, stirring frequently, over medium-high heat until they soften and brown, about 5 minutes. Add vinegar and cook until skillet is almost dry, 4 minutes. Remove pan from heat, stir in remaining butter, pour in any meat juices that have accumulated while steaks rested, season with salt and pepper, and spoon over steaks.