Pre-heat oven to 350 degrees and line a baking sheet with foil.
In a medium bowl, toss together cooked bacon pieces and crumbled feta.
Using a small spoon, fill each mushroom liberally with a teaspoon or two of the bacon/feta mixture. Feel free to pack them high but be careful not to break the mushroom cap.
Bake until warm and slightly bubbly, 9-11 minutes.
Serve immediately or keep in a warm oven until your guests arrive.