Melt butter then saute diced onion and salt on high heat until glassy / translucent look, about 3 - 5 minutes.
Add diced garlic, black pepper and sliced mushrooms. Saute until the mushrooms are collapsed and browned, about 5 minutes.
Stir in optional wine then cook away half of it, for about 5 minutes. Add water.
With and immersion blender puree the soup, then bring it to boil with the lid on.
Remove from heat, mix in sour cream. For serving make croutons or as an even better low carb option, saute mushrooms with garlic and parsley, it's a real treat!