Creamy Meat Balls

Yield: 6 servings

1/4 cup butter
1 medium onion, chopped
2 pound ground beef
2 eggs
2 teaspoons salt
1/2 teaspoon pepper
1/4 teaspoon tarragon
1/4 teaspoon marjoram
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