Make quick tomatillo stock: simmer husks, onion, garlic, jalapeño, and water for 10 minutes; strain and reserve 1/2 cup liquid.
Blend reserved stock with 1/2 the cilantro, lime juice, 1 tbsp olive oil, and salt to make salsa verde.
Preheat oven to 400°F. Cut two 12-inch parchment squares.
Place a fillet on each square, spoon salsa over fish, drizzle remaining oil, and season.
Fold and seal paper into tight packets; bake 12–14 minutes until fish flakes.
Open carefully, garnish with remaining cilantro, and serve right in the packet if you like.