Begin by seasoning the boneless skinless chicken breasts with salt and pepper. Preheat a skillet over medium-high heat for about 2 minutes and add a splash of olive oil.

Carefully place the seasoned chicken breasts in the skillet and cook for 6-7 minutes on each side, until they reach an internal temperature of 165°F.

Once cooked, transfer the chicken to a plate and let it rest for a few minutes.

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

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

Top each with the remaining bread to assemble the sandwich.

Heat a skillet over medium heat with olive oil or butter. Grill the sandwiches for 3-4 minutes on each side.

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