Prep & Marinate: Pound the chicken breasts flat to an even thickness. Combine your barbecue sauce, Frank's RedHot, brown sugar, red onion slices, cooked bacon pieces, and pineapple chunks. Toss with the chicken and marinate for about 4 hours in the refrigerator.
Prep the Grill: Heat up the grill. Lay down a sheet of aluminum foil over the grates (to keep the pineapple chunks and onions from falling through) and spray it with cooking spray.
First Side: Place the marinated chicken breasts onto the greased foil. Spoon some of the sauce, onions, and pineapple chunks directly on top. Close the grill lid and cook covered for about 10 minutes.
Flip & Sauce: Flip the chicken over. Spoon the remaining sauce, bacon bits, and pineapple chunks over the top.
Finish Cooking: Cover the grill again and cook for another 10 minutes, or until the chicken is thoroughly cooked through.
Serve: Transfer to a platter, making sure to scoop up plenty of the caramelized pineapple, red onion, and bacon to serve on top of the chicken breasts.