01 -
Season chicken breasts with salt and pepper. Heat olive oil in a skillet over medium-high heat. Cook chicken 6-7 minutes per side, or until cooked through. Let rest, then slice thinly.
02 -
Slice avocado and prepare cheese slices. Set aside.
03 -
Spread mayonnaise and Dijon mustard on each slice of bread.
04 -
Layer chicken, avocado, cheese, red onion, and tomato on two slices of bread. Top with remaining bread, mayo side down.
05 -
Heat skillet or grill pan over medium heat. Add butter or oil. Cook sandwiches 3-4 minutes per side, until golden brown and cheese is melted. Press down gently with a spatula.
06 -
Remove sandwiches, let cool slightly, slice in half, and serve warm.