Slice each bell pepper in half top to bottom through stem. Remove seeds and membranes. Place in a rectangle 9x13" casserole dish (or two 8x8" to accommodate) that has been sprayed with cooking spray or olive oil. Bake for 25 minutes uncovered.
Heat oil in a large skillet over medium-high heat. Add the onions, garlic, salt and oregano. Cook until soft, 3 minutes. Add ground chicken to hot skillet, break up with spoon.
Cook until no longer pink. Remove from heat and add to large bowl.
Add the mozzarella cheese, drained spinach and cooked rice to seasoned chicken in bowl. Stir to incorporate and heat through. Divide and spoon mixture into each baked pepper, reserving any leftover filling for other use or freeze.
Place in oven to bake 15 minutes to melt cheese and heat through. Serve with squeeze of lemon, parsley crumbled feta cheese if desired.