Preheat oven to 400 degrees.
Carefully butterfly each chicken breast and fold open like a book – be careful not to cut all the way through.
On one half of each chicken breast, layer half a slice of provolone and enough pepperoni to cover (about 4 sandwich size slices). Fold the half of the chicken back over the top.
Season chicken with salt and pepper.
Preheat a 10 inch, oven-safe skillet over medium-high heat. Heat a tablespoon of oil until it shimmers.
Add chicken breasts to pan, top down and cook for 1 to 2 minutes until the top is brown. Carefully flip over, I like to use 2 spatulas but tongs work too.
Transfer the pan to the hot oven. Cook for 5 to 6 minutes. Remove pan from oven and top each chicken breast with a slice of provolone. Return to oven and cook for another 7 to 10 minutes until chicken reaches 165 degrees.
Remove chicken from oven and serve. Garnish with parsley if desired.