Combine the mayonnaise and pesto in a small bowl and stir to combine.
Cut the top off the pepper and slice the pepper in half. Remove the seeds and white pith.
Cover the inside of each pepper half with the pesto mayonnaise and top with provolone. Layer the mushrooms and spinach on top of the provolone, and the basil on top of the spinach. Place final slice of provolone on the pepper half that you are using as the top to help hold the ingredients in and close each half together. Wrap in parchment and use a toothpick to hold in place.