Take the stems off from the mushrooms.
Chop the stems up into small pieces, add to a bowl.
Chop 1/2 yellow onion, sauté in a pan until it gets soft.
Combine sautéd onion, mushroom stems, minced garlic and shredded cheese with 1 Tablespoon of sour cream. Add a small pinch of salt.
Preheat oven to 350F.
Fill the mushroom heads with the mushroom stem filling.
Bake for 30 minutes.