Preheat the oven to 375°F.

Prepare the flank steak: if it is not already butterflied, slice it horizontally to open it flat, then lightly pound to an even thickness (about 1/4–1/2 inch). Season both sides with salt and pepper. Set the steak aside.

Make the cream cheese filling: heat 2 tablespoons oil in a small skillet over medium-high heat. Add 1/2 cup chopped onion and cook 2–3 minutes until tender and starting to turn golden. Add 3–4 cloves minced garlic and cook 1 more minute.

Remove the skillet from the heat and stir in 4 ounces cream cheese until fully combined and smooth. Taste and season with additional salt and pepper if needed. Let the mixture cool slightly (about 2–3 minutes) so it is easier to spread.

Lay the butterflied steak flat on a clean surface and pat it dry. Spread the cream cheese mixture evenly over the steak, leaving a roughly 1-inch border along one long edge.

Evenly layer 2 cups baby spinach over the cream cheese, then top with 8 ounces sliced Provolone (or your chosen melting cheese).

Starting from the long edge with the filling, roll the steak tightly into a log. Secure the roll by tying with kitchen twine at even intervals (about every 1–1.5 inches) or by inserting toothpicks so it holds its shape.

Using a sharp knife, slice the tied roll between the ties into pinwheels about 1–1.5 inches thick. Keep the ties or toothpicks in place to hold each pinwheel.

Heat a large, heavy-duty skillet over medium-high heat. Add any remaining oil from the small skillet, or add a small drizzle of oil to coat the pan. When the pan is hot, sear the pinwheels 2–3 minutes per side until browned.

Transfer the seared pinwheels to a baking dish (or place the oven-safe skillet directly into the oven) and bake at 375°F for 10 minutes.

Remove from the oven, tent loosely with foil, and let rest 5 minutes. Remove twine or toothpicks before serving.