Ingredient Note: Tomatillos look like small green tomatoes covered by a parchment-like husk. Often found in salsas and salads, they can be used raw but cooking enhances the flavor and softens the tough outer skin. Look for them near the tomatoes in markets.
Grill tomatillos, onion slices and jalapenos, turning occasionally, until soft and a bit charred, 10 to 15 minutes. Remove vegetables as they are ready. Let cool for about 10 minutes.
Peel, seed and stem the jalapenos. Place the tomatillos, onions, jalapenos, 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.