Creating the Grilled Chicken Sandwich with Pesto is simple if you follow these steps:

1. Marinate the Chicken: In a bowl, add the chicken breasts, pesto sauce, salt, and pepper. Coat the chicken evenly. Let it marinate for at least 15 minutes for maximum flavor.

2. Preheat the Grill: Heat your grill or grill pan over medium-high heat. Lightly brush the grill with olive oil to prevent sticking.

3. Grill the Chicken: Place the marinated chicken breasts on the grill. Cook for about 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C) and the chicken is no longer pink inside.

4. Toast the Bread: While the chicken is grilling, place the bread slices on the grill for 1-2 minutes per side, until they are golden and crispy.

5. Assemble the Sandwich: Once the chicken is cooked, remove it from the grill and let it rest for a few minutes. Slice the chicken into strips.

6. Layer the Ingredients: On one slice of toasted bread, place a handful of spinach or arugula, followed by grilled chicken strips, sliced tomato, and mozzarella cheese. Top with another slice of bread.

7. Serve: If desired, you can place the assembled sandwich back on the grill for an additional minute to melt the cheese slightly.