Preheat the oven to 350ºF. Spray a baking sheet with olive oil spray.
In a pan on medium-high heat, melt the butter and add the finely chopped mushrooms. Cook for 5 minutes, moving the mushrooms around a few times. Add garlic and cook for an additional 1 minute.
In a large bowl, combine the beef, cooked mushrooms and garlic, salt, grated onion, Worcestershire Sauce, and egg white. Mix to combine.
Cut each pepper in half lengthwise and remove the seeds. Place them in the baking dish. Spoon some of the ground beef mixture into each pepper.
Bake for 15 minutes or until the peppers have softened and the beef is cooked through.
Top the peppers with cheese. Place them back in the oven and bake for additional 3-4 minutes. Top with chopped parsley and marinara sauce or serve more on the side for dipping.
To make ahead of time:Make the ground beef filling and store it in an airtight container in the fridge for 1 or 2 days. The peppers can also be sliced, prepped, and kept in the fridge until you’re ready to put the dish together.You can also stuff the peppers, place them in a covered baking dish, and leave them in the fridge for a few hours until it's time to bake.