Season the chicken breasts generously with salt and pepper on both sides. Let them sit for about 5 minutes.

Heat a tablespoon of olive oil in a skillet over medium-high heat. Place the chicken breasts in the pan.

Cook the chicken for 6-7 minutes on one side, then flip and cook for another 6-7 minutes until the internal temperature reaches 165°F.

Remove the chicken and let it rest for 5 minutes before slicing into strips.

Prepare your other ingredients by slicing the avocado, cheese, red onion, and tomato.

Spread mayonnaise and Dijon mustard on each slice of bread. Layer the chicken, avocado, cheese, onion, and tomato.

In the skillet, add butter or more olive oil. Grill the sandwich for about 3-4 minutes per side until golden brown.

Slice the sandwich in half and serve warm. Consider pairing with sweet potato fries or salad.