Preheat the oven to 375 degrees F (190 degrees C).
Put the rice in a medium oven-proof bowl. Pour in the beef broth and French onion soup. Empty the can of mushrooms into the bowl, and place the stick of butter in without stirring. Cover the bowl with foil, a lid or oven-proof plate.
Bake for 1 hour in the preheated oven. Remove the bowl from the oven and stir. Let stand for a few minutes before serving.
