Sour Cream Meat Balls

1/4 cup butter
1 onion, chopped
2 pounds ground beef
2 eggs
2 teaspoons salt
1/2 teaspoon pepper
1/4 teaspoon pepper
1/4 teaspoon marjoram
2 1/2 tablespoons all-purpose flour
5 1/2 tablespoons tomato paste
3/4 cup beef stock
4 teaspoons Worcestershire sauce
2 teaspoons vinegar
1/2 pound mushrooms sliced
1 cup sour cream

Saut