Heat the oven to 375°F and grease a 9-by-13-inch dish. Cook the noodles 2 minutes short of package directions and drain.
Heat 1 tablespoon oil in a skillet and brown the mushrooms in two batches; transfer them to a plate. Add the remaining oil, beef and onion and cook until the beef is browned. Drain.
Sprinkle in the flour and cook for 1 minute. Stir in the broth, Worcestershire, salt and pepper and simmer until lightly thickened.
Remove from the heat and stir in sour cream, mushrooms, noodles and 1 1/2 cups Swiss. Spread in the dish and top with the remaining cheese.
Bake for 25 to 30 minutes until bubbling. Rest 10 minutes before serving.