Season the chicken breasts with salt and pepper on both sides. Heat a skillet over medium-high heat and add olive oil. Place the seasoned chicken in the skillet and sear for 6-7 minutes on each side until cooked through.

Remove the chicken from the skillet and let it rest for 5 minutes.

While the chicken rests, prepare ingredients by slicing avocado, cheese, red onion, and tomato.

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

In the skillet, melt butter or add olive oil. Place the sandwich in the skillet and grill for 3-4 minutes until golden brown and cheese is melted. Press down slightly.

Flip the sandwich and grill the other side for an additional 3-4 minutes until crispy.

Let the sandwich rest before slicing in half and serve warm.