Crafting Juicy Grilled Flank Steak Rolls is remarkably simple when you follow these steps:

1. Prepare the Marinade: In a bowl, mix olive oil, balsamic vinegar, dried oregano, minced garlic, salt, and pepper.
2. Marinate the Steak: Place the flank steak in a resealable bag and pour the marinade over it. Seal and refrigerate for at least 30 minutes, or up to overnight for more flavor.
3. Prepare the Filling: In a skillet over medium heat, sauté the sliced onion and red bell pepper until tender. Add the chopped spinach and cook just until wilted. Remove from heat and let it cool slightly.
4. Preheat the Grill: Heat your grill to medium-high heat.
5. Assemble the Rolls: Lay the marinated flank steak flat. Evenly distribute the sautéed vegetables and shredded mozzarella cheese on top. Carefully roll the steak up tight from one end to the other.
6. Secure the Roll: Use toothpicks or skewers to secure the rolled steak, ensuring the filling remains intact during grilling.
7. Grill the Rolls: Place the rolls on the grill and cook for about 7-8 minutes on each side, or until the internal temperature reaches 135°F for medium-rare.
8. Rest the Rolls: Remove from the grill and let the rolls rest for about 10 minutes to retain juiciness.
9. Slice and Serve: Crisp ends off the rolls slightly before slicing into pinwheels or rounds for serving, showcasing the beautiful filling.

These steps will lead you to an incredible dish that perfectly highlights the flavors of the grilled flank steak and fillings!