Place the chicken breasts on the sheet pan and spray or brush with olive oil. Season the chicken lightly with salt, pepper, garlic powder, and onion powder.
Bake the chicken for 15-20 minutes or until the internal temperature is 165℉.
Remove the chicken from the oven, cover and allow to cool for at least 10 minutes.
Cut the focaccia buns in half and spread the mayonnaise on the bottom bun.
Place the lettuce, tomato, and onion on the bun.
Slice the chicken breasts and add 1/4 of the chicken per bun.
Add the cheese on top of the chicken, place the top focaccia on the sandwich and serve.