Cook pasta shells according to the package but minus 2 minutes so they are still slightly firm as they will also cook once in the oven, drain and set aside to cool off a bit.
At the same time you can cook the ground chicken in a large frying pan, once cooked add in the peppers, onions and garlic.
Add meat mixture to a large bowl, stir in the ricotta cheese, 1/2 cup of shredded mozzarella, leaving the other 1/2 for topping, 3/4 of the pepperoni. 2 Tbsp parmesan cheese and 2 tsp Italian seasoning, stir everything together well.
Preheat oven to 350F
Place 3/4 cup of pasta sauce in the bottom of a 9x13 casserole dish, I sprayed mine with a little non stick spray..If you try to use a smaller dish all 24 shells will not fit.
Stuff each shell with the meat mixture and lay out in the dish
Top with remaining 1 1/3 cup pasta sauce, 1/2 cup mozzarella, 1/4 of the pepperoni, 1 Tbsp parmesan cheese and 1 tsp Italian seasoning.
Bake in the oven for 25 minutes, let cool for a few minutes before serving, makes 6 servings of 4 pasta shells. This recipe will freeze well.