Preheat oven to 350 degrees Fahrenheit.
Clean each mushroom by using a damp towel to wipe off any dirt.
Remove Stems from the mushrooms. Keep about a 1/3 of the stems and discard the rest.
Heat a pan to medium heat. Dice the bacon and cook until only slightly crispy. Do not overcook. Transfer the cooked bacon to a small bowl.
In the same pan. Add a teaspoon of olive oil. Lower the heat to a low-medium and cook the onions until soft and translucent.
Chop the mushroom stems and add them to the pan with the onions. Cook the mushrooms and onions until the mushrooms begin to soften. Transfer the mushrooms and onions to the bowl with the bacon.
Add the bread crumbs, eggs, chicken stock, dill, parsley, salt, pepper, and onion powder to the bowl.
Stir in the parmesan and mozzarella.
Stuff each mushroom cap with as much of the stuffing as you desire. If you are using 2 (6 ounce) boxes of the Bella Mushrooms, you can use a 10 inch round baking dish. If you are using 3 (6 ounce) boxes of the Bella Mushrooms, you will need a 9 by 13 inch baking dish.
Drizzle the melted butter on and around the mushrooms.
Bake for 20 to 22 minutes.
I used 2 (6 ounce) boxes of the Bella Mushrooms in my recipe. Doing so, you'll have overstuffed mushrooms that looks really cute. However, you could also easily distribute the mixture into 3 (6 ounce) boxes of the Bella Mushrooms.