Season the boneless skinless chicken breasts with salt and pepper. Heat a tablespoon of olive oil in a skillet over medium-high heat. Cook the chicken for 6–7 minutes on each side until it’s golden and no longer pink in the center. Transfer the chicken to a cutting board to rest for a few minutes before slicing it thinly.

While the chicken rests, slice the creamy avocado in half, remove the pit, and scoop out the flesh. Cut it into even slices. Gather your choice of cheese, whether Swiss, cheddar, or provolone, and have it ready to melt on the sandwich.

Take your selected bread and spread mayonnaise on one side of each slice. On the same side, add a layer of Dijon mustard.

On the slices of bread with the mayo and mustard spread, layer the sliced chicken, followed by the avocado, your cheese of choice, and add a layer of red onion and sliced tomato. Top with another slice of bread, mayo-side down.

Heat a skillet over medium heat and add a pat of butter or a drizzle of olive oil. Place the assembled sandwiches in the skillet and cook for about 3–4 minutes per side until golden brown and the cheese is melted.

Once both sides are beautifully golden and the cheese is melted, remove the sandwiches from the skillet. Let them cool for a minute, then slice them in half to reveal the warm, melty goodness inside.