Put water on to boil for pasta. Add 2 T. salt to water. When water comes to a boil, add pasta of your choice, and cook according to the pkg. directions, until al dente. When done, drain and set aside.
In the meantime:
Cook onions in pan using enough margarine so they don't stick to the pan.
When onions are soft, add ground beef. Fry until cooked through. Drain off any excess grease.
Turn to med-high heat, and add mushrooms (drained) and soup, and mix well.
Stir in 2 cups of water, add beef bouillon, and mix into the water. Stir all, and let cook for a few minutes. If it starts bubbling too hard, turn heat down to med.
In a measuring cup, mix 4 T. of constarch with about 1/4 cup of COLD water. When you add this to the mixure, you want the mixture to become thick. (Keep in mind you will be adding sour cream which will thin the sauce, so thicken accordingly). Add more constarch and water mixture, if needed.
Turn heat down to low, and add sour cream. Do NOT BOIL or cook on high, or it will curdle.
Season with pepper and salt, as desired.
Add Kitchen Bouquet, and mix in to get a nice dark color for the sauce.