Heat the oven to 375°F and grease a 9-by-13-inch dish. Cook the noodles 2 minutes short of done and drain.
Brown the mushrooms in 1 tablespoon oil in two batches and transfer them to a plate. Add the remaining oil, beef and onion and cook until browned; drain.
Stir in flour for 1 minute, then broth, Dijon, Worcestershire, salt and pepper. Simmer until lightly thickened.
Remove from heat and fold in sour cream, mushrooms and noodles. Spread in the dish and top with Swiss.
Bake for 25 to 30 minutes, until bubbling and lightly browned. Rest for 10 minutes.