Start by generously seasoning both sides of the boneless skinless chicken breasts with salt and pepper. Allow the chicken to sit for a few minutes.

Heat a tablespoon of olive oil in a skillet over medium-high heat. Cook the seasoned chicken breasts for 6-7 minutes on each side until golden brown and fully cooked through. Transfer to a plate and let it rest for 5 minutes before slicing.

While the chicken rests, slice your ripe avocado, cheese, red onion, and tomato.

Take slices of your choice of bread and spread a layer of mayonnaise and Dijon mustard on one side of each slice. Layer with chicken, avocado, cheese, red onion, and tomato. Top with a second slice of bread, mayo side down.

In the same skillet, add butter or olive oil to coat the bottom. Grill the sandwich for 3-4 minutes on each side until the bread is golden brown and the cheese is melted.

Remove the sandwiches and let them cool for a minute. Slice in half to reveal the layers and serve warm.