Heat the oven to 350°F and butter a 3-quart baking dish with 1 teaspoon of the butter.
Boil the noodles for 2 minutes less than the package directs, then drain them.
Brown the beef in a large skillet over medium-high heat for 7 to 9 minutes, breaking it into crumbles, until it reaches 160°F, then transfer it to a plate.
Melt the remaining butter in the skillet and cook the mushrooms and onion for 8 minutes, until the mushrooms brown and their liquid evaporates.
Sprinkle in the flour and cook for 1 minute, then stir in the broth, soup, Worcestershire, salt and pepper and simmer until slightly thickened.
Remove the skillet from the heat, stir in the sour cream, beef and noodles, and spread the mixture in the dish.
Top with cheddar and bake for 25 to 30 minutes, until bubbling at the center, then rest for 5 minutes.