Season the boneless skinless chicken breasts generously with salt and pepper. In a skillet over medium-high heat, add a splash of olive oil and cook the chicken for 6–7 minutes on each side until golden brown and cooked through. Once cooked, let the chicken rest for a few minutes to retain its juices before slicing.

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

Take two slices of bread, spread mayonnaise and Dijon mustard on one side of each slice. Layer the sliced chicken first, followed by avocado, cheese, red onion, and tomato.

In the same skillet over medium heat, add butter or olive oil. Grill the sandwiches for 3–4 minutes on each side until golden brown and the cheese is hot and melty.

Remove from the skillet and let cool slightly. Slice each sandwich in half and serve warm.