Season two chicken breasts with salt and pepper on both sides. Heat a tablespoon of olive oil in a skillet over medium-high heat. Add the chicken and cook for 6–7 minutes per side until golden brown. Remove from heat and let rest for 5 minutes before slicing thinly.

Slice up a ripe avocado, half a red onion, and a juicy tomato. Prepare your choice of cheese by slicing it to fit within the sandwich.

Spread mayonnaise and Dijon mustard on one side of each of the two slices of bread.

On the unspread side of one slice, layer sliced chicken, avocado, cheese, red onion, and tomato. Top with the second slice of bread, mayo side down.

In the skillet, add olive oil or butter and grill the sandwich for 3–4 minutes until golden. Carefully flip the sandwich and grill the other side for another 3–4 minutes until cheese is melted.

Remove from skillet and let sit for a minute before slicing in half to serve.