Pound out chicken breasts to about 1 inch thick. Season with salt, pepper, and garlic. Cover and put in the refrigerator.
Combine the pineapple juice, soy sauce, brown sugar, and mustard in a small saucepan. On medium-high heat, whisk until it becomes a glaze, about 3-4 minutes. Set aside.
Heat grill to medium-high heat, about 350℉.
Spray the chicken with cooking spray before placing it on the grill. This will help it not stick to the grill. Cook for 3-4 minutes, then flip.
Cover the cooked side with glaze and cook for 3-4 minutes. Flip, continue to glaze. Cook until the internal temperature reaches 165℉. (Total cook time is about 10-13 minutes).
Place the cheese slice on the chicken. When melted, remove from the heat. You can also grill buns if you like them toasted.
While the cheese is melting, place pineapple rings on the grill to obtain grill marks and heat the pineapple. Place the grilled pineapple on top of the melted cheese.
Place 1/2 Tablespoon mayonnaise on each side of the buns and add the grilled chicken, and serve.