In a large skillet, melt the butter on medium heat. Add onion and cook for about 15 minutes until softened. Stir often. (Or, set slow cooker to saute and complete this task in the main cooker.)
Add tomatoes (do not drain), wine, sugar and tarragon into the crock pot. Add onion and butter. Cover and cook on Low for 5 hours.
Puree using an Immersion Blender
or in a blender in batches. Season with salt. Serve with cold sour cream.