01 -
Pat the chicken breasts dry and season both sides evenly with salt and ground black pepper.
02 -
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Once hot, add the chicken breasts and cook for 6 to 7 minutes per side until the internal temperature reaches 165°F and the chicken is no longer pink. Remove from heat and rest for 5 minutes before slicing thinly across the grain.
03 -
Slice the avocado, cheese, red onion, and tomato to desired thickness. Set aside for assembly.
04 -
Evenly spread mayonnaise and Dijon mustard on one side of each bread slice.
05 -
On two slices of prepared bread, arrange sliced chicken, avocado, and one cheese slice each. Top with red onion and tomato. Cover with the remaining bread slices, ensuring the mayo side is facing in.
06 -
Melt butter or heat olive oil in a skillet or grill pan over medium heat. Place the assembled sandwiches in the pan and cook for 3 to 4 minutes on each side until bread is golden brown and cheese has melted, pressing gently with a spatula for even contact.
07 -
Remove sandwiches from the pan, let stand for 1 minute, then slice in half and serve immediately.