Prepare the chicken. Slice the chicken in half horizontally. Use a mallet and pound the chicken until it's about ¼ inch thick. Make the marinade for the chicken. Combine garlic with rosemary, lemon zest, lemon juice, olive oil, salt, and pepper. Pour over chicken, tossing to coat. Let sit for 30 minutes.
Meanwhile, prepare the pepperoncini aioli. Combine all the ingredients together in a small bowl until well combined. Season with salt and pepper and set aside.
Heat 1 tbsp oil in a saute pan over medium heat. Add the chicken and cook until browned, about 5 minutes. Flip the chicken and cook another 4-5 minutes or until the juices run clear. Remove from heat and let sit for 5 minutes.
Assemble the sandwiches. Spread 1 tbsp pepperoncini aioli on one side of 8 of the bread slices. Top 4 slices of bread with lettuce, 2 slices tomato, and rosemary lemon chicken. Top with the remaining 4 slices of bread with the aioli side facing up. Stack on the bacon, avocado, 2 slices tomato, and remaining bread. Slice in half and serve.