Set oven to 425°F (220°C). Line a sheet pan with parchment and dust lightly with semolina or cornmeal.
Squeeze thawed spinach in a clean towel until nearly dry. Sauté with 1 tablespoon olive oil and garlic for 1–2 minutes. Cool slightly.
In a bowl, combine spinach, mozzarella, Parmesan, oregano, red pepper flakes, salt, and pepper. Stir in 1/4 cup marinara—just enough to bind without making it wet.
On a lightly floured surface, roll dough into a 12×16-inch rectangle, about 1/8–1/4 inch thick. Trim edges if wildly uneven.
Smear remaining 1/4 cup marinara thinly over dough, leaving a 1-inch border on the long far edge. Distribute filling evenly.
Starting at the long edge nearest you, roll tightly like a jelly roll. Pinch the seam to seal. Turn seam-side down.
Using a sharp knife or dental floss (pro move), cut into 20–24 pieces, about 3/4 to 1 inch thick.
Place cut-side up on the prepared pan, slightly spaced. Brush tops with remaining olive oil or egg/milk wash for sheen.
12–16 minutes until puffed, golden, and cheese is bubbly. Rotate pan halfway for even browning.
Sprinkle with extra Parmesan and a pinch of oregano. Let rest 5 minutes—molten cheese is hot, FYI.
With warm marinara for dipping. Try a drizzle of chili oil if you like heat.