Season the chicken breasts with salt and pepper on both sides.

Heat olive oil in a skillet over medium-high heat. Add the chicken and cook for about 6-7 minutes per side, or until fully cooked through. Once done, let them rest for a few minutes before slicing thinly.

While the chicken is resting, slice your avocado, red onion, and tomato. Prepare the cheese slices and set aside.

Take your sliced bread and spread mayonnaise and Dijon mustard on each slice.

On two of the bread slices, layer the sliced chicken, avocado, cheese, red onion, and tomato. Top with the remaining bread slices, mayo side down.

In a skillet or grill pan heated over medium heat, add butter or olive oil. Cook the sandwiches for 3-4 minutes per side, pressing gently with a spatula until they are golden brown and the cheese is melted.

Remove from heat, let cool for one minute, slice in half, and serve warm.