In a heavy saucepan, bring the water to a boil. Add the tomatillos, 3/4 of the sliced onion, 6 of the chiles, and 6 of the garlic cloves. Simmer the mixture covered for about 30 minutes, or until the tomatillos are tender. Cool slightly and transfer to a blender or food processor. Add the remaining onion, chiles, and garlic, cilantro, and salt. Puree the mixture. Preheat the oven to its lowest heat and heat a well-seasoned flat griddle pan or cast iron skillet over medium heat. Brush each slice of cheese with a little oil, then grill the slices for about 20 seconds on each side. You will need a heavy metal spatula to turn them, scraping all the golden crust from the pan and flipping it over. You will have to grill the cheese in batches, transferring the cooked cheeses to a platter in the oven while you finish cooking, but do not let the cheese melt. Place 2 slices of cheese on each of 6 warmed appetizer plates, pour the hot tomatillo sauce over the top and serve immediately, garnished with sprigs of cilantro and charred scallions. Accompany with wedges of toasted country bread.