Step 1: Prepare the Chicken

1. Season the chicken breasts with salt and pepper on both sides.
2. Heat the olive oil in a skillet over medium-high heat.
3. Cook the chicken breasts for about 6-7 minutes per side, or until fully cooked and no longer pink in the center.
4. Remove from the skillet and let rest for a few minutes before slicing thinly.

Step 2: Prepare the Avocado and Cheese

1. Slice the avocado and cheese and set them aside.

Step 3: Prepare the Bread

1. Spread a thin layer of mayonnaise and Dijon mustard on each slice of bread.

Step 4: Assemble the Sandwiches

1. On two slices of bread, layer the sliced chicken, avocado, and cheese.
2. Add a few slices of red onion and tomato on top.
3. Top each sandwich with the remaining slices of bread, mayo side down.

Step 5: Grill the Sandwiches

1. Heat a skillet or grill pan over medium heat and add a bit of butter or olive oil.
2. 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.
3. Press down gently with a spatula to help the cheese melt evenly.

Step 6: Serve and Enjoy

1. Remove the sandwiches from the skillet and let them cool for a minute before slicing in half.
2. Serve warm and enjoy!