Cut the peppers in half from top to bottom so you have 4 halves. Remove and discard the seeds and membranes.
Place the peppers on a baking sheet (or directly on the oven rack) and bake for 10 minutes.
While the peppers are cooking, in a large skillet, heat the oil over medium heat. Add onion and cook about 3 minutes. Stir in diced zucchini and cook until tender, about 5 minutes more. Stir in chicken, salsa, and salt & pepper to taste.
Simmer 5-7 minutes or until thickened. Spoon the filling into the peppers.
Place the peppers in the bottom of a 9x9-inch baking dish.
Cover and bake for 25 minutes. Remove foil, top with cheese, and bake uncovered for an additional 10 minutes or until cheese is melted and peppers are tender.
Notes
Store leftover chicken stuffed peppers in a covered container in the fridge for up to 4 days.
Reheat in the microwave or freeze them first on a baking sheet, then move to zip-top bags for up to 2 months.
Thaw in the fridge or warm them in a covered skillet with a splash of water until hot.