Making the Grilled Chicken Panini Sandwich is straightforward if you follow these simple steps:

1. Marinate Chicken: In a bowl, combine olive oil, balsamic vinegar, Italian seasoning, salt, and pepper. Add the chicken breasts, ensuring they are well-coated. Allow to marinate for at least 15 minutes.
2. Preheat Grill or Panini Press: Heat your grill or panini press to medium-high heat.
3. Grill Chicken: Remove the chicken from the marinade and grill for 6-7 minutes on each side or until the internal temperature reaches 165°F (75°C). Once cooked, let it rest for a few minutes before slicing.
4. Prepare Bread: While the chicken is resting, slice the ciabatta or sourdough bread.
5. Assemble Sandwich: On one slice of bread, layer a slice of provolone cheese, fresh spinach, sliced tomato, avocado, and sliced grilled chicken. Top with another slice of cheese and the second slice of bread.
6. Grill Panini: Place the assembled sandwich on the preheated grill or panini press. Cook for 3-5 minutes, or until the bread is golden and the cheese has melted.
7. Slice and Serve: Remove the sandwich from the grill, let it cool for a minute, then slice it in half to serve.

These steps will guide you in creating this incredible sandwich with ease.