Roast poblanos on an open flame or under a broiler, turning regularly until the skin is evenly blistered and blackened, about 5 minutes on an open flame or 10 minutes under a broiler. Be careful to char only the skin, not the flesh. Cover with a towel and let stand for 5 minutes. Rub off blackened skin, then pull or cut out stems and seed pods. Tear chiles open and quickly rinse to remove stray seeds and bits of skin. Cut into 1/4-inch-wide strips about 2 inches long.
In a sauté pan, cook chorizo over medium heat, stirring to break up any clumps, about 5 minutes or until half-cooked. As chorizo heats, it should render enough fat to cook the meat; if the mixture seems dry, add a little oil. Add onions and cook about 10 minutes, stirring frequently, until onions are richly golden and chorizo is cooked. (If the mixture looks very oily, drain.) Stir in poblano strips and season with salt. Mixture can be made ahead and refrigerated; reheat to order.
For each serving, to order, heat 1/3 cup chorizo mixture in a small cazuela. Stir in 1/3 cup cheese. Bake in 350°F oven about 5 minutes or until cheese is just melted but has not begun to separate or look greasy. Sprinkle with 1/4 teaspoon oregano and serve immediately with 2 tortillas.
Recipe courtesy of Rick Bayless, Frontera Grill and Topolobampo, Chicago, Ill.
