Generously season your chicken breasts with salt and pepper. Heat a skillet over medium-high heat and add the olive oil. Allow it to heat up until shimmering.

Carefully place the seasoned chicken in the hot skillet. Cook for 6-7 minutes on each side until golden brown and cooked through. Let it rest for a few minutes to retain juices.

While the chicken is resting, slice the avocado, cheese, red onion, and tomato. Set aside for assembly.

Spread mayo and Dijon mustard on one side of each bread slice. Layer the chicken, avocado, cheese, onion, and tomato on two slices, topping with the other slices, mayo side down.

Heat a skillet or grill pan and add butter or oil. Grill sandwiches for 3-4 minutes per side until golden brown and cheese melts.

Remove from skillet and let cool for a minute before slicing. Serve warm.