Season the boneless skinless chicken breasts generously with salt and pepper. Heat 1 tablespoon of olive oil in a skillet over medium-high heat. Cook for 6-7 minutes on each side until golden brown and fully cooked. Let it rest before slicing.

While the chicken rests, slice the ripe avocado, cheese, red onion, and tomato. Prep two slices of your favorite bread, whether whole grain or a baguette.

Spread mayonnaise on one slice of bread and Dijon mustard on the other if using. Layer sliced chicken, avocado, cheese, red onion, and tomato. Top with the second slice of bread.

Heat a clean skillet over medium heat, adding 1 tablespoon of butter or olive oil. Grill the sandwich for 3-4 minutes until golden brown, then flip and grill the other side for another 3-4 minutes.

Once both sides are golden and cheese is gooey, remove from the skillet. Cool for a minute, slice in half, and serve warm.