Season the chicken breasts generously with salt and pepper. In a skillet, heat 1 tablespoon of olive oil over medium-high heat until shimmering. Cook the chicken for about 6-7 minutes on each side or until it’s golden and no longer pink in the center. Once cooked, remove from the skillet and let it rest for a few minutes before slicing.

While the chicken rests, slice the ripe avocado, cheese, red onion, and tomato into thin pieces.

Take your slices of sourdough or ciabatta bread and spread mayonnaise on one side of each piece. On the opposite side, spread Dijon mustard.

On one slice of bread, layer the sliced chicken, followed by the creamy avocado, cheese, red onion, and tomato. Top with another piece of bread, mayo side down.

In the same skillet, heat another tablespoon of butter or olive oil over medium heat. Carefully place the assembled sandwiches in the skillet and grill for about 3-4 minutes on each side until golden-brown.

Once grilled, transfer the sandwiches to a cutting board. Allow them to cool slightly, then slice each sandwich in half and serve warm.