Preheat the oven to 350° F.
On a parchment-lined baking sheet, Roast the tomatillos for 30 minutes, or until very soft but not split. Coarsely chop them.
In a large saucepan, combine the tomatillos, onion, vegetable stock, garlic, chili, vinegar, cumin, and sugar. Bring to a boil. Reduce the heat and simmer, covered, for 20 minutes. Add the chopped cilantro, and transfer to a blender or food processor. Pureé until smooth.
Season to taste with salt.
Yields: 3 cups