Step 1: Prepare Chicken - Start by seasoning boneless skinless chicken breasts with a generous pinch of salt and pepper, ensuring even coverage. Let them sit for about 10 minutes to absorb the flavors.

Step 2: Cook Chicken - Heat a skillet over medium-high heat and add a drizzle of olive oil. Once the oil shimmers, carefully place the seasoned chicken breasts in the skillet. Cook for 6–7 minutes on each side or until the chicken is golden brown and no longer pink in the center. Remove from the heat and allow the chicken to rest for a few minutes.

Step 3: Prep Ingredients - While the chicken is resting, slice your ripe avocado, cheese, red onion, and tomato into thin pieces.

Step 4: Assemble Sandwich - Spread a mixture of mayonnaise and Dijon mustard generously on one side of each bread slice. On the unspread side, layer the cooked chicken, avocado slices, your choice of cheese, and a few onion and tomato slices. Top with the other slice of bread.

Step 5: Grill Sandwich - Preheat your skillet over medium heat and add a pat of butter or a splash of oil. Carefully place the assembled sandwiches in the skillet and grill for 3–4 minutes per side.

Step 6: Serve - Once grilled to perfection, remove the sandwiches from the skillet. Let them cool for a minute before slicing them in half.