Stir together chicken, onion, BBQ sauce, shredded cheese, egg, and salt.
Stir in 1/2 cup breadcrumbs or cornflakes. The amount you need will depend on which of those you use and how thick your sauce is. Add up to 1/2 cup more as needed until the meat can hold together into patties.
Divide meat mixture into four portions, roll each portion into a ball, and then flatten into a patty.
Spray large skillet with cooking spray, and heat over medium-high heat. Place burger patties into hot skillet, and cook for approximately 7 minutes per side, or until cooked through.
Place a slice of cheese over each burger to melt for the last 2-3 minutes of cooking (if desired). Spread a light layer of mayonnaise on both sides of the buns, add cooked patties, and top with spinach and tomato. Enjoy!