Slice enough mushroom caps to make 2 cups; finely chop stems and remaining mushrooms.
In a large saucepan, saute sliced mushrooms in 1 tablespoon margarine until browned, about 5 minutes, remove and reserve.
Saute onion and chopped mushrooms in remaining 1 tablespoon margarine in saucepan until onion is tender, about 5 minutes.
Add stock and cups milk and heat to boiling.
Mix remaining 1/2 cup milk and cornstarch and stir into boiling mixture; boil, stirring frequently, until thickened, about 1 minute. Stir in reserved slice mushrooms. Season to taste with salt and pepper.