Season both sides of the chicken breasts generously with salt and pepper. Heat olive oil in a skillet over medium-high heat and add the chicken. Cook for 6-7 minutes on each side, or until the internal temperature reaches 165°F.

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

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

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

Heat a skillet over medium heat and grill the assembled sandwiches for 3-4 minutes on each side until the bread is golden brown and the cheese is melted.

Remove from heat, allow to cool slightly, then slice in half and serve warm.