Heat the margarine in a large skillet over medium-high heat (350 degrees in an electric skillet). Add onion and mushrooms. Sauté and stir until starting to brown.
In a small bowl, mix water, bouillon, garlic powder and cornstarch. Add to skillet and cook until sauce is thickened.
Remove from heat. Stir in sour cream.
Serve with cooked whole grain noodles or brown rice. Garnish with paprika and chopped parsley if desired.