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.