Step 1

Melt butter in a skillet over medium heat. Add onion and mushrooms. Reduce heat to low and cover skillet. Cook for 15 minutes. Stir, cover again, and cook until tender, about 15 minutes more.

Step 2

Pour enough water into the skillet to moisten the mixture. Add bouillon; stir until dissolved. Cut bacon into small pieces over the sauce. Simmer, uncovered, for 10 minutes.

Step 3

Increase heat to medium and bring sauce to a simmer. Add sour cream and cornstarch; stir constantly for 2 minutes until sauce is blended and thickened. Remove from heat.