In a large 3 quart sauce pan, melt butter over medium heat. Add onions and saute until soft. Add garlic and cook for another minute. Add tomatoes and their juices, stock, and water. Bring to a simmer then remove from heat. Let cool for 10 minutes.
Add one-half of the sour cream. Whisk until well-blended, then add the remaining sour cream.
Whisk until sour cream is combined.
Stir in pepper and cheese to blend. Add additional salt and pepper to taste.
Gently reheat to serve.