Prepare the Chicken:
Season chicken breasts with salt and pepper on both sides. Heat olive oil in a skillet over medium-high heat. Cook chicken for about 6–7 minutes per side, until cooked through and no longer pink. Remove from skillet and let rest a few minutes, then slice thinly.

Prep the Avocado and Cheese:
Slice avocado and have cheese ready.

Prepare the Bread:
Spread mayonnaise and Dijon mustard on one side of each bread slice.

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

Grill the Sandwiches:
Heat butter or olive oil in a skillet over medium heat. Place sandwiches in the skillet and cook for 3–4 minutes per side, pressing gently with a spatula, until bread is golden and cheese is melted.

Serve:
Remove sandwiches from the skillet. Let cool slightly, slice in half, and serve warm.