Grill tomatillos, onion slices and jalapeños, turning occasionally, until soft and a bit charred, 10 to 15 minutes. Let cool for about 10 minutes.
Peel, seed and stem the jalapeños. Place the tomatillos, onions, jalapeños, lime juice, vinegar, brown sugar and salt in a food processor. Pulse until the mixture is well blended but still has a chunky texture. Serve warm or chilled.