Lay the flank steak on a cutting board with the grain running left to right. If not already butterflied, slice horizontally through the thickest part, stopping before cutting through, and open like a book. Cover with plastic wrap and pound to an even 1/2-inch thickness with the meat mallet. Season both sides with 1/2 teaspoon salt and all the pepper.

Heat olive oil in a large skillet over medium heat. Add garlic and cook 30 seconds until fragrant. Add spinach in batches, tossing until just wilted, about 2 minutes total. Transfer to a plate and let cool 5 minutes. Squeeze out excess moisture with paper towels.

Spread the cooled spinach evenly over the steak, leaving a 1-inch border on all sides. Scatter sun-dried tomatoes and red pepper flakes if using. Lay provolone slices in a single layer, slightly overlapping.

Starting from a short end, roll the steak tightly away from you, keeping the filling tucked in. Tie with kitchen twine at 1.5-inch intervals to secure. Wrap in plastic and refrigerate 15 minutes to firm up, or up to 8 hours ahead.

Preheat oven to 375F. Remove twine and slice the roll into 8 even pinwheels, about 1-inch thick. Heat a grill pan or large oven-safe skillet over medium-high heat. Sear pinwheels 2-3 minutes per side until well-browned.

Transfer pan to oven and roast 10-12 minutes until internal temperature reaches 130F for medium-rare. Rest 5 minutes before serving. The cheese should be melted and bubbling, the steak juicy with a faint pink center.