Add the mushrooms to the pot with the other onions. Simmer for about five minutes.  Add the soup and water simmer for one minute. Add sour cream and milk, stir until smooth. Simmer on low, adding the balls as they come out of the oven.  Check for spice.  You may need to adjust the salt, pepper, or garlic.
All of the meatballs should cook for a minimum of 5 more minutes with the sauce.
Always taste and check a dish prior to serving.