Clean mushrooms well. Remove stems and carve out a portion of the center of each mushroom. Finely chop stems and carvings and set aside.
In a large skillet, sauté mushroom caps for 2 to 4 minutes in 2 tablespoons of the butter or oil. Remove caps and place hollow-side up on a silicon mat or parchment-lined baking sheet.
In the same skillet, on high heat, add remaining butter or oil. Sauté onions for few minutes to brown, stirring constantly.
Turn heat to medium-low and continue to cook the onions until caramelized, stirring occasionally.
Add chopped mushrooms and seasonings to the skillet. Cook 5 more minutes.
Stir in bread crumbs and cheese.
Fill caps with stuffing. Place under preheated broiler for 5 minutes or until nicely browned.
Serving Size: 24