01 -
Generously season both sides of the chicken breasts with salt and freshly ground black pepper.
02 -
Heat olive oil in a skillet over medium-high heat. Sear the chicken breasts, cooking each side for 6-7 minutes until thoroughly cooked and no pink remains. Remove from heat and allow to rest for a few minutes.
03 -
Once rested, slice the chicken thinly. Slice avocado and prepare cheese, onion, and tomato.
04 -
Spread a thin layer of mayonnaise and Dijon mustard on each slice of bread. Top two slices with sliced chicken, avocado, cheese, onion, and tomato. Cover with remaining bread, mayo-side down.
05 -
Heat butter or olive oil in a skillet or grill pan over medium heat. Place the assembled sandwiches in the pan, cooking for 3-4 minutes per side, pressing gently with a spatula, until the bread is golden brown and the cheese is fully melted.
06 -
Remove sandwiches from the pan and let stand for 1 minute. Slice each sandwich in half and serve warm.