With a sharp knife, butterfly chicken breasts into two halves, being careful not to cut completely in half.
Slice red onion into thin slices and mushrooms into slices
Season interior of chicken breast with garlic parmesan rub.
Stuff butterfly chicken breast with 1 slice of cheese, spinach leaf, red onion, and mushroom slices.
Roll closed and place closed side down on the baking sheet
Bake at 400 for 40 minutes
Let rest for 10 minutes after removing from the oven.
Serve with a side of veggies or a side salad.