1. Preheat a grill or grill pan to medium-high heat.

2. Season the boneless chicken with salt and pepper, and drizzle with olive oil.

3. Grill the chicken for about 5-6 minutes on each side, until fully cooked. Set aside to rest.

4. While the chicken is resting, slice the bread and brush with olive oil. Toast the bread on the grill for about 1-2 minutes on each side, until lightly charred.

5. In a small pan, sauté the diced onion until caramelized and set aside.

6. Assemble the sandwich by placing the grilled chicken on one slice of bread, followed by a slice of American cheese. Top with cherry tomatoes and caramelized onions. Place the other slice of bread on top.

7. Serve hot and enjoy!