Prepare the chicken by cutting the chicken breast in half crosswise, then set the thinner bottom portion aside. Cut the thicker top portion of the breast in half lengthwise so you have three equal-sized pieces of chicken per breast.
Place the pieces, one at a time, in a heavy-duty zip-lock bag and pound gently to a ¼-inch thickness using a meat pounder.
Marinate the chicken by returning all pieces to the zip-lock bag and pour 1-1½ cups of teriyaki sauce. Seal the bag and squish it around to coat the chicken. Refrigerate for 2-6 hours.
Prepare the caramelized onions by heating the butter in a large skillet over medium heat. Add the diced onions once the butter has melted; stir often. Stir the onions frequently, adding a tablespoon of water to prevent them from drying out, until they are caramelized and deeply golden brown, about 30 minutes. Set aside to cool.
Make the spicy sauce by combining the mayonnaise and sriracha sauce until well combined. Set aside to allow the flavors time to mingle.
Cook the chicken by heating a grill pan over medium heat. Add the pieces of chicken and cook for 8-10 minutes, flipping over halfway through cooking and brushing with some of the teriyaki sauce.
Add the Swiss cheese and cook for a minute or two until melted. Remove and place on a serving plate.
Place the pineapple slices on the grill pan and cook for a minute or two on each side.
To serve, spoon some caramelized onions on each piece of chicken.
Place one on each piece of chicken.
Toast the buns, then slather with spicy sauce.
Place the chicken on the bun and top with lettuce and tomatoes. Serve immediately. Enjoy.