Generously season the boneless skinless chicken breasts with salt and pepper. Allow the chicken to sit for about 10 minutes.

Heat a skillet over medium-high heat and add a splash of oil. Cook the chicken for 6-7 minutes on each side until golden brown and internal temperature reaches 165°F. Let the chicken rest for 5 minutes.

While the chicken rests, slice the avocado, cheese, red onion, and tomato. Lay the sliced ingredients aside.

Spread mayonnaise and Dijon mustard on one side of each bread slice. Layer the chicken, sliced avocado, cheese, red onion, and tomato, then cover with another slice of bread.

Heat a clean skillet over medium heat and add a pat of butter. Place the assembled sandwich in the skillet and grill each side for about 3-4 minutes until golden brown and cheese is melted.

Remove the sandwich from the skillet, let it cool for a minute, then slice in half.