For the tomatillo sauce: Lay the tomatillos, garlic, and chiles on a baking sheet and place about 4 inches below a very hot broiler. When they have blackened on one side, about 5 minutes, turn them over and roast the other side. You may need to remove the garlic and chiles sooner. Once the garlic has cooled, peel them, then transfer the peeled garlic along with the cilantro, 2 serranos (reserve the last one for the mousse), tomatillos, and any accumulated juices to a food processor or blender. Pulse to reduce the ingredients to a smooth puree. Add the cream and 1 teaspoon salt, pulse a few more times to blend. Pour into a resealable container and refrigerate until ready to use. This can be done several days in advance.
For the seafood mousse: Arrange the mahi mahi and shrimp in a single layer on a lined cookie sheet and place in the freezer for 30 minutes. Refrigerate the food processor bowl and blade while the seafood is chilling so that they will also be cold. After 30 minutes, scoop the mahi mahi, shrimp, serrano, cilantro and 3/4 teaspoon salt into the chilled processor bowl and run it for about 30 seconds or until the mixture is finely chopped. Add the egg white and cream, then continue processing for another 30 to 45 seconds until you have a smooth paste. Scrape the mixture into a bowl and refrigerate for at least 1 hour. You can prepare this mousse early in the day.
For the vegetables: Heat a very large (12-inch) non-stick skillet with the oil over high heat. Once the oil is hot, scoop in the mushrooms and sauté until lightly brown, about 5 minutes. Lower the temperature to medium, stir in the leeks and 1/2 teaspoon salt, then sauté for an additional 10 minutes until the leeks are completely soft. Let cool to room temperature. You can refrigerate this mixture for several days if you want to do this step in advance.
For the mahi mahi: Preheat the oven to 300 degrees with the rack set in the center.
Unfold the banana leaves, cut 4 – 15-inch pieces choosing sections of the largest relatively unbroken leaves. Trim off the thin hard strip of the side of all the pieces.
To make the pieces pliable, either steam them for 20 minutes or pass each one briefly over an open flame until it turns from dull and stiff to soft and shiny.
Lay the soft banana leaf pieces out on the counter, divide the mushroom/leek mixture evenly between the 4 pieces. Spread the mixture out in the center of the leaf to form a ring leaving the middle clear. This is where you’ll place the mahi mahi. Season both sides of each fillet with salt, then place one fillet on each of the leaves. Spread equal portions of the mousse over the entire top surface of the fillets. Shingle 3 or 4 raw tomatillo slices over the top of each piece. Fold one end of the banana leaf over the top of the fish, then fold in the sides, finally fold the other end over the top. Secure the package by threading a skewer through the top folds. Place the mahi mahi packages onto a rimmed cookie sheet and slide into the oven. They'll need to cook for between 35 to 40 minutes.
To serve: Just before the mahi mahi is done, pour the tomatillo sauce into a small saucepan. Rewarm over low heat. Once warm, taste, and adjust seasoning, if necessary. You may also need to add a few tablespoons of water to bring the consistency of the sauce to that of a light cream soup.
Place each package into a rimmed plate, slide out the skewer and fold back the banana leaf to expose the mahi mahi. Ladle the warm tomatillo sauce around the fish and sprinkle the tops with the chopped cilantro. Pour the remaining sauce into a bowl and pass around to your guests.
Jill’s Wine Recommendation: The crisp Mohua Sauvignon Blanc from Marlborough, New Zealand would be great with this. I like how the tomatillo pairs with the guava and passion fruit notes in the wine.