Season the chicken breasts with salt and pepper on both sides. Heat olive oil in a skillet over medium-high heat and cook for 6–7 minutes per side until fully cooked. Let it rest a few minutes before slicing thinly to retain juices.
Slice your avocado and cheese while the chicken rests. Keep them ready to assemble so the sandwich goes together quickly and stays fresh.
Spread a thin layer of mayonnaise and Dijon mustard on each slice of bread. This adds moisture, flavor, and a barrier to prevent the bread from becoming soggy.
Layer chicken slices, avocado, cheese, tomato, and red onion on two bread slices. Top with the remaining bread slices, mayo side down. Press gently to hold everything together.
Heat butter or olive oil in a skillet over medium heat. Grill sandwiches 3–4 minutes per side until golden brown and cheese melts, pressing lightly with a spatula for even melting.
Remove sandwiches from the pan, let cool slightly, and slice in half. Serve warm for maximum gooeyness and flavor.