PreparationMake tomato sauce: Preheat broiler. Broil tomatoes and chiles in a foil-lined small shallow baking pan 2 to 3 inches from heat, turning frequently, until lightly charred on all sides, 3 to 5 minutes for chiles, 12 to 15 minutes for tomatoes (flesh should be soft). Alternatively, char vegetables in a hot well-seasoned griddle or cast-iron skillet over moderate heat. Core tomatoes and stem chiles, then purée with onion and 1/2 teaspoon salt, or to taste, in a blender until smooth (use caution when blending hot ingredients). Heat oil in a nonstick skillet over moderately high heat until just hot. Stir 1/2 cup tomato sauce into oil (this reduces spattering), then, when bubbling vigorously, add remaining sauce. Fry, stirring, until slightly thickened, about 3 minutes. Remove from heat and stir in cilantro and salt to taste. Prepare memelas: Stir together masa harina, 1/2 teaspoon salt, and water in a large bowl with a wooden spoon until it forms a dough. Knead in bowl with heel of hand 1 to 2 minutes. Divide dough into 16 pieces and roll each into a 1 1/2-inch ball. Roll balls into 3-inch-long logs. Transfer logs as formed to a plastic-wrap–lined tray and cover with more plastic wrap. Heat a well-seasoned large cast-iron griddle or skillet over moderately high heat until hot. Cut 2 (5-inch) squares from a small plastic bag and use 1 to line lower half of tortilla press. Put a log of dough in press and top with second square of plastic. Close press, pushing gently on lever to flatten dough to a 4- by 3- by 3 1/2-inch oval. If press doesn’t flatten evenly, rotate tortilla 180 degrees and gently flatten again. (Alternatively, use a rolling pin to flatten dough between squares of plastic.) Remove plastic and cook oval on hot griddle until a skin forms and some small golden spots appear, about 1 1/2 minutes per side. Make more ovals, cooking 3 or 4 at a time. Remove memelas from heat and, when cool enough to handle, form sides: Beginning 1/2 inch from edge of each memela and working all around it, push undercooked masa toward edge with your thumb and pinch up to form a 1/4-inch side. Then pinch up masa in center of memela in several places. If dough still seems very raw, cook memelas upside down on hot griddle 30 seconds to 1 minute. Sprinkle about 2 tablespoons cheese on each memela and top with 1 to 2 tablespoons tomato sauce. Heat filled memelas, 4 at a time, on hot griddle until bubbling, about 3 minutes. Serve hot. Cooks' notes:·Sauce may be made 1 day ahead and chilled, covered. Add cilantro just before using.·Memelas may be prepared, without adding filling, 4 hours ahead, cooled completely, then chilled in a sealed plastic bag.