Prepare the Chicken: Season the chicken breasts with salt and pepper on both sides. Heat olive oil in a skillet over medium-high heat. Cook the chicken for about 6-7 minutes per side or until fully cooked. Remove from heat and let rest before slicing thinly.

Prepare the Avocado and Cheese: Slice the avocado and set aside with the cheese slices.

Prepare the Bread: Spread a thin layer of mayonnaise and Dijon mustard on each slice of bread.

Assemble the Sandwiches: Layer the sliced chicken, avocado, and cheese on two slices of bread. Add red onion and tomato slices. Top with the remaining slices of bread, mayo side down.

Grill the Sandwiches: Heat a skillet or grill pan over medium heat. Add a bit of butter or olive oil to the pan. Grill the sandwiches for 3-4 minutes per side until the bread is golden and the cheese is melted.

Serve: Remove from skillet and let cool slightly. Slice in half and serve warm.