Preheat the oven to 375 degrees F. Spray a 9x13 baking pan lightly with oil (or use an ungreased cast iron pan). Place the chicken in the pan.
Combine the salt, pepper and garlic powder in a tiny bowl. Season the chicken by pinching the spice mixture between your fingers and sprinkling it evenly on each piece of chicken.
Place 2 tablespoons of BBQ sauce on each breast and spread the sauce over the chicken with the back of your spoon.
Shred the mozzarella if needed. Sprinkle about 2 ½ tablespoons of shredded mozzarella over the entire surface of each breast, using all of the cheese.
If starting with a whole red onion, peel it and cut it in half. Start making thin slices and place 5 or six slices of onion on each chicken breast. (You'll use about ⅓ of a whole onion.)
Bake the chicken for 25 minutes, uncovered. Check to see if it's done by using an intstant meat thermometer to see if it has reached 165 degrees F. If you don't have a thermometer, cut open a thick piece to make sure it's no longer pink inside. If needed, bake for another five minutes before checking it again. 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. Store leftovers in the refrigerator for up to four days. This also freezes well.
Notes
Chicken BreastsUse thick, boneless, skinless chicken breasts. I haven't tested this recipe with cutlets, which would cook faster. You should be able to substitute boneless, skinless chicken thighs, but you won't have as much surface area for the toppings.See the article with this recipe for other substitutions.BBQ SauceUse your favorite store-bought or homemade BBQ sauce.