Although you can broil the fish, it is at its best when cooked outside (placing the fish in a grill basket makes it easier to turn over on the barbecue). Ask the fishmonger to remove the head and scales and to clean the fish.
2 pounds ripe tomatoes
3 serrano chiles, stemmed
¼ cup coarsely chopped white onion
3 garlic cloves, chopped
¼ cup chopped fresh cilantro
1 tablespoon chopped fresh parsley
1 ½ teaspoons fine sea salt
4 whole Snapper, head and scales removed, fish cleaned
¼ cup canola oil
1 teaspoon dried Mexican oregano
2 teaspoons fresh lime juice
12 5- to 6-inch-diameter warm corn tortillas
Line heavy large skillet with foil. Place tomatoes and chiles in skillet; cook over medium-high heat until skins begin to blister and blacken in spots, turning occasionally, about 20 minutes for tomatoes and 10 minutes for chiles. Remove from heat and cool slightly. Peel tomatoes; transfer to processor. Add chiles, onion, and garlic. Blend until coarse puree forms. Transfer sauce to bowl. Stir in cilantro, parsley, and salt. (Can be prepared 4 hours ahead. Cover and chill.)
Prepare barbecue (medium-high heat) or preheat broiler. Rinse fish inside and out with cold water; pat dry. Open fish flat, like a book. Rub fish inside and out with oil; sprinkle inside with oregano, salt, and pepper. If grilling, place fish, opened flat, in grill basket. If broiling, place fish, opened flat, on oiled rimmed baking sheet.
Grill or broil fish just until opaque in center, about 6 minutes per side.
Using spatula, transfer fish to platter. Sprinkle with lime juice; serve with tomato sauce and warm corn tortillas.