Add mushrooms to same
skillet and toss to coat with oil. Cook, stirring occasionally, until softened, about 4 minutes. Season with salt and cook, stirring occasionally, until starting to brown, about 4 minutes. Reduce heat to medium and add butter, reserved green beans, and 1½ tsp. kosher salt. Cook, tossing often and spooning foaming butter over vegetables, until butter darkens slightly and smells nutty, about 5 minutes. Add
vinegar and season generously with pepper. Cook, stirring often, 1 minute.