Begin by butterflying the flank steak. Lay it flat on a cutting board, and with a sharp knife, carefully slice it horizontally, creating a pocket without cutting all the way through.
In a skillet over medium heat, add the oil. Once hot, sauté the chopped onion until translucent, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant. Remove from heat and let cool slightly.
In a bowl, mix the sautéed onions and garlic with the cream cheese, sliced provolone, and baby spinach. Stir until well combined, and season with salt and pepper to taste.
Spread the filling evenly over the butterflied flank steak. Leave a little space around the edges to avoid overflow when rolling.
Starting at one end, carefully roll the flank steak tightly around the filling. Secure the roll with toothpicks or tie it with kitchen twine to keep everything in place.
Preheat your oven to 375°F (190°C). Place the stuffed rolls seam-side down in a baking dish. Bake for about 25-30 minutes, or until the steak reaches your desired doneness. Let rest for 5 minutes before slicing.
Slice the rolls into thick pieces and serve warm. Enjoy with your favorite side dish or a fresh salad for a complete meal.