Preheat oven to 350°. Line a heavy sheet pan with parchment paper, and lay a cooling rack on top of the parchment.
Clean the mushrooms with a damp paper towel. Gently remove the stems and set aside for the filling.
Place the empty mushroom caps, open side up on the cooling rack, and put the pan in the preheated oven, for about 15 minutes.
Meanwhile, make the filling. Chop the mushroom stems into about 1/4 inch pieces.
Melt the butter in a large saute pan, over medium heat and add the mushrooms, onions, and garlic. Sprinkle with the salt and pepper and saute until the onions are translucent and soft.
Raise the heat to medium-high, and continue to cook off the excess liquid - about 5 minutes. Stir every minute or two.
Remove the pan of mushrooms from the oven [don't turn the oven off]. Suspend the rack, with the mushrooms on it over the sink [unless you have a very large sink, the rack should sit on it ok]. Turn over each cap and allow them to drain for about 5 minutes. You can spray the bottoms with avocado oil spray, which will prevent them from looking too dry.
Add bread crumbs to the skillet and stirring every so often, continue to cook until the bread crumbs begin to toast.
Remove from the heat, and stir in the cheese.
Place the mushrooms, open side up on the parchment-lined sheet.
Use a teaspoon to fill the bottom of each mushroom cap, making sure to get the filling all the way in the corners. Then use something like a small cookie scoop to mound the filling on top. Press the filling down a bit, You want them filled, but not so much that they are top-heavy.
Return the pan to the oven and continue to bake for about 20 to 25 minutes, until the bread crumbs look nicely toasted.
Serve immediately or these can be kept warm in a low oven for about 30 minutes.
Notes
Refrigerate leftovers. They can be reheated in the oven.