Melt butter in a skillet over medium heat. Stir onion and mushrooms into melted butter. Reduce heat to low and cover the skillet. Cook for 15 minutes. Stir, cover again, and cook until tender, about 15 minutes more.
Pour enough water into the skillet to moisten onion-mushroom mixture. Add bouillon; stir until dissolved. Cut bacon into small pieces over sauce. Simmer, uncovered, for 10 minutes.
Increase heat to medium. Add sour cream and cornstarch; stir constantly until sauce is blended and thickened, about 2 minutes.