Butterfly and flatten the flank steak by laying it on a cutting board and slicing it open like a book. Keep it in one piece by stopping half an inch before fully cutting through. Cover with plastic wrap and gently pound until evenly thin.

In a bowl, mix the softened cream cheese, chopped spinach, minced garlic, grated parmesan, shredded mozzarella, salt, pepper, and red pepper flakes (if using). The filling should be thick and spreadable.

Spread the filling over the steak, leaving a small border around the edges. Roll tightly from the long side, then tie every couple of inches with kitchen twine. Chill for 20 to 30 minutes.

Preheat an oven-safe skillet with oil. Sear the steak roll on all sides until browned.

Transfer the skillet to a preheated 375°F oven and cook until the center reaches your preferred doneness (135-140°F for medium).

Let the roll rest for 10 minutes before slicing into thick pinwheels.