Season the chicken breasts generously with salt and pepper. Heat a skillet over medium-high heat and add olive oil. Once hot, cook the chicken for 6–7 minutes per side until golden brown and cooked through. Let rest before slicing.

Slice the avocado in half, remove the pit, and scoop the flesh out. Cut into thin slices. Prepare your choice of cheese by slicing it.

Spread mayonnaise on one side of each slice of bread, adding a smear of Dijon mustard for extra flavor.

Layer sliced chicken, avocado, cheese, red onion, and tomato on half of the bread slices, then top with remaining slices.

Heat skillet over medium heat, add butter or olive oil, and grill sandwiches for 3-4 minutes per side until golden brown and cheese is melted.

Allow the sandwiches to rest for a minute before slicing in half and serving warm, optionally with a side of salad.