Season the chicken breasts with salt and pepper on both sides.
Heat the olive oil in a skillet over medium-high heat.
Cook the chicken breasts for about 6-7 minutes on each side, or until fully cooked and no longer pink in the center. Remove from the skillet and let rest for a few minutes before slicing thinly.
Slice the avocado and prepare the cheese slices. Set aside.
Spread a thin layer of mayonnaise and Dijon mustard on each slice of bread.
On two slices of bread, layer the sliced chicken, avocado, and cheese. Add a few slices of red onion and tomato on top.
Top each sandwich with the remaining slices of bread, mayo side down.
Heat a skillet or grill pan over medium heat. Add a bit of butter or olive oil to the pan.
Place the sandwiches in the skillet and cook until the bread is golden brown and the cheese is melted, about 3-4 minutes per side. Press down gently with a spatula to help the cheese melt evenly.
Remove the sandwiches from the skillet and let them cool for a minute before slicing in half. Serve warm.
If you somehow manage to have leftovers, store your sandwich in an airtight container in the refrigerator. When reheating, use a toaster oven or regular oven to maintain the bread’s crispiness. Microwaving can make the bread soggy, so it’s best to avoid it if possible.
