Remove and finely chop the mushroom stems. Set aside. Using a spoon, scrape out the gills (the black part on the underside of the mushroom cap) and discard. Set aside.
In a large bowl or freezer bag, combine all the ingredients. Add the mushroom caps and coat well. Marinate for about 10 minutes.
With the rack in the middle position, preheat the oven to 200°C (400°F). Line a baking sheet with aluminum foil or parchment paper.
Fry the bacon in a skillet over medium heat until browned. Add the chopped mushroom stems, asparagus, bell pepper and onion. Season with salt and pepper. Sauté over medium-high heat for about 2 minutes. Add the broth. Bring to a boil and reduce until almost dry. Add the croutons and mix well. Set aside.
Remove the mushroom caps from the marinade and arrange them on the cookie sheet. Season with salt and pepper. Fill with the stuffing and sprinkle with the cheese. Roast until the cheese is lightly browned, about 20 minutes.
Serve as a starter or as a side dish for grilled meats.