Preheat your oven to 375 degrees F. Spray your baking pan lightly with oil and place the chicken in the pan.
Season your chicken with salt, pepper and garlic powder.
Place 2 tablespoons of BBQ sauce on each breast and spread the sauce over the chicken with the back of your spoon.
Shred your mozzarella if needed. Sprinkle 2 ½ tablespoons of shredded mozzarella on each breast.
If starting with a whole red onion, peel it and cut it in half. Start making thin slices and place 4-5 slices of onion on each chicken breast. (You'll probably use ⅓ of your whole onion.)
Bake the chicken for 25 minutes, uncovered. (Don't overbake, or the chicken will be dry.) Let the chicken rest on the stove, covered loosely with foil, for a few minutes before cutting into it. This will keep it juicy and allow it to finish cooking. The chicken should no longer be pink inside, but if it is, bake it a few minutes longer.