De-stem, and then roast the serrano peppers in a broiler oven until the skins blacken. Turn, repeat. Remove to cool. In a medium skillet, sautee the onions in oil, adding the granulated sugar as you go, until they turn a caramel brown. You may, at this point, deseed the serrano peppers to reduce their spiciness, but I don't find a need to do that. Put the caramelized onions and the serrano peppers into the food processor and pulse until it is a smooth paste. Remove using a rubber spatula to a container. Husk the tomatillos and chop them coarsely. Puree the salsa ingredients in food processor until smooth, mixing them into a ~6 quart bowl. Add chile paste to the bowl to desired spiciness, mixing well. 2 teaspoons per pound of tomatillos is about right. Freeze leftover chili paste in airless ziploc bags.