Bring a large pot of water to a rolling boil. Add cut peppers with seeds removed to the pot and boil for about 3 minutes.
Using tongs, drain and then remove peppers from the pot to a baking dish.
Preheat oven to 350 °F.
If using leftover meatloaf, then simply crumble meatloaf with a fork in a bowl and skip steps 5, 6, and 7.
Combine oatmeal, 1 cup mozzarella cheese, Parmesan cheese, diced onion, and basil in a large bowl. Mix these ingredients together before adding the ketchup and egg.
Add ketchup and egg to the bowl and mix to thoroughly combine. It's easier to evenly distribute all of the ingredients if you mix the ingredients in this order.
Add ground chicken and mix just until combined. Don't overmix though or the chicken will toughen while cooking.
Scoop ½ cup of the chicken mixture into each pepper with the tops cut off. If the pepper has been cut lengthwise, then use ¼ cup. Baby bell peppers generally take about 2 Tablespoons each to fill.
Top with additional cheese and the top of the pepper if you're using it.
Bake uncovered for 30 minutes or until the internal temperature reaches 165 °F. The exact cook time will vary based upon the way you cut the peppers, how big they are and whether or not the meat has been cooked first. See notes below on timing. It will take about 30 minutes for peppers cut in half through the stem and when the meat it already cooked.
Allow stuffed peppers to cool for a few minutes and enjoy!