Season the boneless skinless chicken breasts generously with salt and pepper. In a skillet, heat 2 tablespoons of olive oil over medium heat. Cook the chicken for 6–7 minutes on each side until golden brown and cooked through. Transfer to a plate and let it rest for 5 minutes before slicing thinly.

While the chicken rests, slice the avocado into thin pieces. Prepare your chosen cheese by slicing or grating as needed. Slice the red onion and tomato into thin rounds, setting aside for easy assembly later.

Take 4 slices of your favorite sturdy bread and spread mayonnaise on one side of each. Add a thin layer of Dijon mustard atop the mayonnaise side.

On the mayo/mustard side of 2 bread slices, layer the sandwich with sliced chicken, avocado slices, cheese, onion, and tomato. Top with the remaining bread slices, mayo side down.

In the same skillet, add 1 tablespoon of butter or olive oil and heat over medium. Place the assembled sandwiches in the skillet and grill for 3–4 minutes on each side.

Remove the sandwiches from the skillet and let them cool for a moment. Slice each sandwich diagonally and serve warm.