Move oven rack about 4-inch below broiler element, preheat broiler to high heat.
Place tomatillos and jalapeno on a baking sheet then place in oven and roast about 4 – 5 minutes until charred on top (keep a close on on things as they can char pretty quickly – especially the pepper).
Rotate veggies to opposite side and roast until charred on opposite side, about 3 – 5 minutes longer.
Slice pepper then add to a food processor along with onion, cilantro, garlic, salt, sugar, lime juice, water and tomatillos (and any juices on pan).
Pulse several times to a coarse puree. Stir in more water to thin if desired. Chill through in refrigerator in an airtight container.