Preheat oven to 450 degrees F. Line a rimmed baking sheet with foil or parchment and spray with cooking spray.
Halve each chicken breast horizontally to create 2 cutlets. Cover with a piece of wax paper and lightly pound to a 1/4 in thickness. Season with salt and pepper.
Lay the chicken cutlets on a work surface and top each piece, in the center, with 1 tablespoon mozzarella, 2 slices pepperoni, and 2 slices mushroom. Loosely roll each one up and set them seam side down.
In a bowl, combine the bread crumbs and Parmesan. In a second bowl, combine the olive oil, lemon juice, and pepper, to taste.
Dip a chicken roll-up into the oil mixture, then in the bread crumb mixture. Place it seam side down on the prepared baking sheet. Repeat with the remaining chicken. Lightly spray the roll-ups with cooking spray.
Bake until the chicken is nearly done, about 20 minutes. Remove the baking sheet from the oven.
Top each roll-up with 1 tablespoon marinara, 1 tablespoon mozzarella, 2 slices red onion, and 1 1/2 teaspoons bell pepper.
Bake until the cheese is melted and the sauce is hot, 4 to 5 more minutes. Serve immediately.
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