Preheat the oven to 350° F.
Twist to remove mushroom stems and chop. Set aside.
In a saucepan over medium heat, add a tablespoon of bacon grease or olive oil. Sauté onion for 5 minutes or until tender and translucent.
Add chopped mushroom stems and garlic to saucepan and cook 3 minutes.
Reduce heat to low and stir in bacon, cream cheese, parmesan and pecan pieces.
Stuff mushrooms with mixture and place in baking dish. Bake 20 minutes.
Salt and pepper to taste.