Cover cleaned and roughly chopped mushrooms with water in a large saucepan. Bring to a boil and then immediately remove from heat and drain completely.
Chop raw bacon into 1 inch pieces. Fry until crisp. Drain off most of the fat, leaving some to sautee the mushrooms and onion. A tablespoon or two will do.
Add the onion to the bacon and remaining grease. Cook over medium heat until they start to become translucent. Add the well drained mushrooms and saute for a couple of minutes, stirring occasionally.
Sprinkle with flour and cook for another minute or two, stirring frequently, to cook the flour taste out. Stir in the milk and cook until it starts to thicken.
Remove from heat and stir in the sour cream. Adjust your salt and pepper to your liking.