You're likely familiar with skirt and flank steaks. This is better. Flap, from the bottom sirloin, makes butchers and cooks smile because its tenderness-to-flavor ratio is ideal. Almost-high heat will facilitate the crust you need while allowing the steak to develop a more end-to-end "eye." The sugar-salt-chile rub becomes an irresistible crust on this now on-the-radar cut. Serve this with First-Cut Salsa Fresca and Maria's Cilantro Cebollas.