Preheat oven to 400 degrees F.

Prepare breadcrumbs first by heating a small saucepan over medium-low heat and adding 2 tablespoons of butter. Add in 2 minced garlic cloves and cook for 30 seconds until fragrant, then stir in bread crumbs well, tossing for a minute or two until the mixture is combined and slightly golden. Set aside.

Heat a large skillet oven medium heat and add bacon. Cook until all fat is rendered and bacon is crispy, stirring often. Remove bacon with a slotted spoon and place to drain on a paper towel. To the same skillet, add in onion and garlic with salt and pepper, cooking for 2 minutes. Stir in spinach and artichokes, cooking for 5-6 minutes until spinach is fully wilted. Reduce heat to low and stir in mascarpone cheese. Once melted, add in bacon, 4 ounces of the cheddar cheese and all of the parmesan. Stir well until combined and melted, then turn off heat.

Place the mushrooms stem side up on a baking sheet. Stuff each with an equal amount of the spinach mixture, then top each with the remaining grated cheddar. Top that with the garlic butter breadcrumbs. Bake for 20-25 minutes, until cheese is bubbly and golden. Serve immediately.