Lay the butterflied flank steak on a clean surface and pound it to an even thickness. Season the steak with minced garlic, chopped parsley, chopped oregano, salt, and ground black pepper.
Layer the sliced hard-boiled eggs, red bell pepper, julienned carrot, spinach leaves, and grated Parmesan cheese evenly over the steak.
Starting from one end, carefully roll the steak tightly into a log shape. Secure the roll with kitchen twine at 1-inch intervals to keep the filling in place.
In a small bowl, whisk together the olive oil and white wine vinegar. Place the rolled steak in a shallow dish and pour the marinade over it. Cover and refrigerate for at least 2 hours, or overnight for best results.
Preheat your grill to medium-high heat, or preheat your oven to 375°F (190°C).
If grilling, place the rolled steak on the grill and cook for about 20-30 minutes, turning occasionally, until the meat is browned on all sides and cooked to your desired doneness.
If baking, place the rolled steak in a baking dish and bake in the preheated oven for about 30-40 minutes, or until the meat is cooked through.
Remove the matambre from the grill or oven and let it rest for about 10 minutes. Carefully remove the kitchen twine and slice the matambre into 1-inch thick pieces.
Arrange the sliced matambre on a serving platter and serve hot. It can be enjoyed with chimichurri sauce, a fresh salad, or grilled vegetables.
Enjoy your meal!