01 -
Season chicken breasts with salt and pepper. Heat olive oil in skillet over medium-high heat. Cook chicken 6-7 minutes per side until fully cooked. Let rest before slicing thinly.
02 -
Slice avocado and prepare cheese slices. Set aside.
03 -
Spread thin layer of mayonnaise and Dijon mustard on each slice of bread.
04 -
On two bread slices, layer sliced chicken, avocado, cheese, red onion, and tomato. Top with remaining bread slices, mayo side down.
05 -
Heat skillet over medium heat with butter or oil. Cook sandwiches until golden brown and cheese melts, about 3-4 minutes per side, pressing gently with spatula.
06 -
Remove from skillet, let cool briefly, slice in half, and serve warm.