01 -
Season chicken breasts with salt and pepper on both sides. Heat olive oil in a skillet over medium-high heat. Cook chicken for about 6–7 minutes per side, until cooked through and no longer pink. Remove from skillet and let rest a few minutes, then slice thinly.
02 -
Slice avocado and have cheese ready.
03 -
Spread mayonnaise and Dijon mustard on one side of each bread slice.
04 -
On two slices of bread, layer sliced chicken, avocado, cheese, red onion, and tomato. Top with the remaining bread slices, mayo side down.
05 -
Heat butter or olive oil in a skillet over medium heat. Place sandwiches in the skillet and cook for 3–4 minutes per side, pressing gently with a spatula, until bread is golden and cheese is melted.
06 -
Remove sandwiches from the skillet. Let cool slightly, slice in half, and serve warm.