Winter Tarragon Tomato Soup

1 1/2 cups chopped onion
1/2 cup butter 
1 quart canned tomatoes or 2 pounds peeled frozen tomatoes or 2 pounds peeled fresh tomatoes 
1/2 cup dry white wine 
1 tablespoon sugar 
1 teaspoon dried tarragon 
salt to taste 
sour cream or creme fraiche 

In a large saucepan over medium heat, saute the onion in the butter until it starts to turn gold, about 15 minutes. Add the tomatoes, juice and all, and mash them down with a potato masher. Add the wine, sugar, and tarragon and stir. Cover, turn the heat down and simmer the soup for 45 minutes. Puree the soup and push it through a sieve back into the saucepan. Add salt to taste. Serve with a dollop of sour cream in each bowl.

Serves 4.