Preheat the oven to 450°F (230°C). Line a large baking sheet with foil or parchment and lightly spray with oil.
Halve each chicken breast horizontally to make two thin cutlets. Place a piece of wax or parchment paper over each and gently pound with a meat mallet or rolling pin to about 1/4-inch thickness.
Season both sides of each flattened cutlet with 1/4 teaspoon kosher salt (divide evenly) and freshly ground black pepper to taste.
Arrange the fillings on the center of each cutlet: about 1 tablespoon shredded mozzarella, 3 slices turkey pepperoni, and 3 slices white button mushroom per cutlet.
Loosely roll each cutlet around the fillings and place seam-side down on the prepared baking sheet.
In a small bowl, mix the bread crumbs and grated Parmesan. In another small bowl, whisk together 1/2 tablespoon olive oil and 1 tablespoon lemon juice, seasoning with pepper to taste.
Dip each chicken roll-up first into the oil-lemon mixture, then roll in the breadcrumb-Parmesan mixture to coat. Return to the baking sheet seam-side down and lightly spray the tops with oil.
Bake the roll-ups until the chicken is almost cooked through, about 20 minutes.
Remove the baking sheet from the oven. Top each roll-up with an even spoonful of marinara sauce, the remaining shredded mozzarella, red onion slices, and chopped green bell pepper.
Return to the oven and bake until the cheese is melted and sauce is hot, about 4–5 minutes. Serve immediately.
