Coarsely chop the tomatillos, four unblemished ones for the garnish. Place in a food processor or blender. Chop three-quarters of the onion, three-quarters of the cucumber, the garlic, poblano chiles and the cilantro and add to the food processor. Puree, adding ice water to thin if necessary. Add salt, then chill thoroughly.

For salsa garnish, very finely mince the remaining tomatillos, onion, cucumber and serranos. Season with salt and chill.

To serve, taste again for salt and add more ice water if too thick. Place in chilled bowls, add a dollop of crème fraiche or sour cream, and a tablespoon or ore of the salsa. Garnish with line wedges.