Begin by seasoning boneless skinless chicken breasts with salt and pepper. Heat a skillet over medium heat and add a splash of olive oil. Cook the chicken for 6-7 minutes on each side until browned and fully cooked through. Remove from skillet and let rest for a few minutes before slicing.

While the chicken rests, prepare the rest of your ingredients by slicing the avocado, red onion, tomato, and cheese.

Take two slices of your chosen bread, spread mayonnaise on one side of each slice, and then add Dijon mustard on one slice.

Layer sliced chicken, avocado, cheese, red onion, and tomato on the bread. Top with the second slice of bread, mayonnaise side down.

Heat a skillet or grill pan and add butter or olive oil. Grill the sandwich for about 3-4 minutes on each side until golden brown and cheese is melted.

Remove from skillet, let cool briefly, then slice in half and serve warm.