Preheat oven to 350°. Spray foil lined baking sheets with non-stick cooking spray
Wash mushrooms, remove stems and set the stems aside . ( *The stems easily pop off when you gently pull them to the side). Finely chop HALF the stems.
With a small spoon, gently scoop out mushrooms so there is room for stuffing. Add mushroom scrapings to finely chopped stems.
In a microwave safe bowl, cook butter, onion, garlic and only HALF mushroom stems and scrapings from the mushroom for 4 minutes. You can save the other half, eat or discard in the compost bin.
In a medium bowl, mix together instant mashed potatoes, cheese, bacon bits, sauerkraut, sour cream, salt, pepper. * Eliminate bacon bits if making vegetarian.
Then add the onion, mushroom stem, butter garlic mixture and stir until combined.
Stuff mushrooms,using a small spoon, then sprinkle with parmesan cheese, dividing evenly between all the mushrooms.
Bake until for 15 - 20 minutes or until mushrooms are golden and cooked through.