How to Make It
In a small saucepan, sprinkle the tomatillos with a pinch of salt. Cover with water, bring to a boil and cook just until the tomatillos change color, 5 minutes. Drain and let cool. Transfer the tomatillos to a blender, add the lime juice, onion, jalapeños, cilantro sprigs, garlic cloves and 1/4 cup of water and puree.
In a large nonstick skillet, heat the olive oil. Season the fish inside and out with 1 teaspoon of salt. Add the fish to the skillet and fry over moderately high heat, turning once, until golden and crisp, 15 minutes.
Tilt the pan slightly and spoon off the oil. Add the sauce and cook, spooning the sauce over the fish, until it is hot and the fish is cooked through, 5 minutes. Using 2 spatulas, transfer the fish to a platter. Add the butter to the skillet; swirl until melted. Season the sauce with salt, pour over the fish and serve.