2. Remove and discard the stems and seeds from the chilies. Toast in a dry heavy skillet over medium heat until fragrant, about 5 minutes. Transfer to a bowl, pour on the hot water, and let soften for 30 minutes. Lift out chilies and discard the water.
3. Place the tomatillos on a baking sheet and roast for 20 minutes. Let cool and place in a blender and process to a purée. Transfer to a bowl and set aside.
4. In the blender, combine the chilies and 1/2 cup of fresh water and process to purée.
5. In a large heavy saucepan, heat the oil over medium heat. Stir in the flour, garlic, and cumin and cook for 2 to 3 minutes. Remove from heat. Pour the chili mixture through a sieve into the flour mixture. Stir in the tomatillos, lemon juice, and sugar. Return to low heat and simmer for 10 minutes. Remove from heat and transfer to a serving dish.