Season the chicken breasts with salt and pepper. Heat olive oil in a skillet over medium-high heat. Cook chicken for 6-7 minutes on each side until golden brown and fully cooked.

While chicken rests, slice the avocado, prepare cheese, and slice red onion and tomato if using.

Spread mayonnaise and Dijon mustard on one side of each slice of bread.

Layer chicken, avocado, cheese, and optional red onion and tomato between two slices of bread.

In the skillet, add butter or olive oil, heat, and grill sandwiches for 3-4 minutes on each side until golden and cheese is melted.

Let grilled sandwiches cool for a minute before slicing them in half. Serve warm.