Season the chicken breasts with salt and pepper. Heat olive oil in a skillet over medium-high heat. Add the chicken and cook for 6-7 minutes on each side until no longer pink. Let rest before slicing.

While the chicken rests, slice the avocado, red onion, and tomato into thin rounds.

In a small bowl, mix mayonnaise and Dijon mustard until smooth. Spread the mixture on the inside of each slice of bread.

Layer bread with sliced chicken, avocado, onion, tomato, and cheese. Press the top slice of bread to secure the layers.

In the same skillet, add butter or olive oil over medium heat. Grill the sandwich for 3-4 minutes per side until golden brown and cheese is melted.

Allow sandwiches to rest before slicing in half. Serve with your favorite side.