Remove papery husks from tomatillos and rinse well. Clean onions; wash thoroughly. Cut the tomatillos and onions in half, and place in medium bowl with olive oil. Toss to coat.
Place onions and tomatillos cut side down on a foil-lined baking sheet. Place under a broiler for about 5-7 minutes to lightly blacken the skin.
In a blender or food processor, place tomatillos, lime juice, onions, cilantro, garlic, chili peppers, sugar and one large ice cube, and pulse until all ingredients are finely chopped and mixed. Season to taste with salt. Chill in refrigerator.