Season boneless skinless chicken breasts generously with salt and pepper. Preheat your skillet to medium-high heat.

Add a drizzle of olive oil to the hot skillet. Grill the seasoned chicken for 6-7 minutes until browned, then flip and cook for another 6-7 minutes until fully cooked through.

While the chicken rests, slice the avocado, cheese, red onion, and tomato.

Spread mayonnaise and Dijon mustard on one side of each slice of bread. Layer with sliced chicken, avocado, cheese, red onion, and tomato.

Grill the sandwich in a preheated skillet with butter or olive oil for 3-4 minutes per side until golden brown and cheese melts.

Remove the sandwich from the skillet, let it cool for a minute, slice in half, and serve warm.