Season chicken breasts with salt and pepper. Heat olive oil in a skillet over medium-high heat. Cook chicken for 5–7 minutes per side, until cooked through and juices run clear. Remove and let rest, then slice.

In a small bowl, mix Dijon mustard and mayonnaise.

If preferred, mash the avocado in a bowl and season lightly with salt and pepper.

Butter one side of each bread slice (or brush with olive oil).

Assemble two sandwiches: On the unbuttered side, spread Dijon-mayo, then layer sliced chicken, avocado, red onion, tomato, and cheese. Top with the second bread slice, buttered side facing out.

Heat a clean skillet or griddle over medium heat. Grill sandwiches for 3–4 minutes per side, pressing gently, until bread is golden brown and cheese is melted.

Remove from heat, let rest for 1 minute, then slice in half and serve immediately.