Make salsa: Purée tomatillos, chiles, onion, garlic, salt, and water in a blender until smooth.
Heat oil in a 10-inch heavy skillet over moderate heat until hot but not smoking, then carefully add tomatillo purée (oil will splatter) and simmer, stirring, until thickened, 8 to 10 minutes. Stir in cilantro and simmer 1 minute. Transfer to a small bowl.
Make chalupas: Put oven rack in middle position and preheat oven to 200°F.
Heat 1 tablespoon oil in cleaned dried skillet over moderate heat until hot but not smoking, then briefly cook tortillas, 3 at a time, until softened and hot but still pale, about 10 seconds per side, adding more oil if necessary. Transfer tortillas with tongs to paper towels to drain briefly, then to a large shallow baking pan in oven to keep warm, arranging tortillas in one layer (overlapping slightly if necessary) and covering pan with foil.
Spread each warm tortilla with about 1 teaspoon salsa, then top sparingly with a few shreds of chicken. Thin crema with a little water and drizzle over chicken, then sprinkle with onion and cheese. Serve immediately.
Cooks' notes: If you can't find 4-inch tortillas, you can buy larger ones and trim them using a 4-inch cookie cutter or a paring knife. Salsa can be made 2 days ahead. Cool, uncovered, before chilling, covered. Reheat to warm.