Dress up pizza dough with a topping of Pecorino Romano cheese and caramelized onions to create this crowd-pleasing onion focaccia.
Preheat oven to 425 degrees. Coat an 11-by-17-inch rimmed baking sheet with 2 tablespoons oil. Place dough on sheet, stretching to fit (if dough springs back, let rest 20 minutes and repeat). Drizzle edges with 1 tablespoon oil. Cover with plastic wrap and let sit 15 minutes.
Meanwhile, in a large skillet, heat1 teaspoon oil over medium-high. Addonions and season with salt and pepper.Cook, stirring occasionally, until onionsare tender and golden brown in spots,12 minutes. Add vinegar and cook,stirring and scraping up browned bitswith a wooden spoon, 1 minute.
Remove plastic from dough. Withyour fingertips, poke dimples all overdough, then top with cheese, brownedonions, and red-pepper flakes. Bakeuntil golden brown around edges, about30 minutes. Let cool on sheet 5 minutes.Cut into 20 pieces and serve warmor at room temperature.