Husk and halve 1 pound of tomatillos. Place them cut-side down on a grill until browned and softened; flip and repeat. Brush 3 ears of corn with olive oil and grill about 12 minutes, rotating frequently.
In a food processor, combine tomatillos, ½ cup chopped red onions, ½ cup cilantro, 3 chopped garlic cloves, juice of 2 limes, juice of ½ an orange, 1 teaspoon cumin, and ½ cup water. Process until smooth.
Carefully remove corn from the cobs using a sharp knife. Add to tomatillo purée. Stir in 1 each finely diced jalapeño and serrano peppers, 1 tablespoon salt, and one teaspoon pepper. Serve with chips or on top of fish.