Step 1:
Start by marinating the chicken breasts. In a small bowl, combine olive oil, balsamic vinegar (if using), Italian seasoning, salt, and pepper. Coat the chicken breasts in the marinade and let them rest for 10-15 minutes.

Step 2:
Heat a grill pan or panini press over medium heat. Once it’s hot, place the marinated chicken breasts on the grill. Cook the chicken for 6-7 minutes per side, or until the internal temperature reaches 165°F (75°C). Remove the chicken from the heat and let it rest for a few minutes before slicing.

Step 3:
While the chicken is resting, prepare the panini bread. Spread a thin layer of mayonnaise or pesto on one side of each slice of bread. This will add flavor and help the bread crisp up when grilled.

Step 4:
Slice the grilled chicken into thin strips. Place the chicken slices on two slices of bread. Top with cheese slices and, if desired, fresh tomato slices.

Step 5:
Place the other slice of bread on top, with the mayonnaise or pesto side facing down. Preheat your panini press and grill the sandwich for about 4-5 minutes, or until the bread is golden brown and the cheese is melted.