In large skillet, add bacon pieces, cook until done. Set aside bacon. Cook onions in bacon fat until cooked. Set aside with bacon. Allow grease to drain as much as possible. Place each of the mushroom caps into a mini-muffin tin and set aside.
In a small mixing bowl, add cream cheese, garlic, and pepper. Combine ingredients well. Add onion and bacon to mix. Take a plastic storage back and place the bacon mixture into it, and push all the mixture down into one corner. Snip the corner of the bag to allow the bacon mixture to be pushed through. Squeeze the bacon mixture in to each of the mushroom caps, until all caps are filled.
Place the mushrooms in the oven for 10-12 minutes until bacon mixture is light brown. Enjoy!