Fry bacon over medium heat until crisp. Remove from pan, and blot with paper towels. When cool, chop or break it into 1-inch pieces.
Meanwhile, remove stems from mushroom caps. Gently toss caps in tablespoon of avocado oil.
Preheat oven to 375 F. Line a baking sheet with parchment paper.
In a food processor, chop bacon finely. Add garlic and onion; chop again. Add mushroom stems and spinach; chop finely. Add oatmeal, cheese, salt and pepper and pulse just until blended. If you don't have a food processor, chop the ingredients finely by hand and stir them together.
Stuff filling into mushroom caps, piling extra on top. Press it in place a little with your fingers and set each one onto prepared baking sheet.
Bake for 20 minutes or until cooked through and cheese is melted.