Steaming fish in a parchment packet in the oven, cooking en papillote as the French call it, yields deeply flavorful results and is a messproof way to cook fillets.
Preheat oven to 450 degrees. Fold two 13-by-19-inch sheets of parchment in half from short ends to create a crease in each; unfold. Place each fillet to one side of a sheet, abuttingcrease; generously season with salt and pepper. Scatter mushrooms, asparagus, and onion evenly around fillets. Drizzle each fillet with 2 tablespoons wine. Scatter butter evenly over fish and vegetables; season with salt and pepper.
Fold each sheet over fish and vegetables. Starting on one side, roll bottom side of parchmentup and over top side to seal; continue working around until pouch is sealed in a half-moonshape. Transfer both pouches to a rimmed baking sheet.
Steam-roast until parchment puffs and darkens slightly and fish is just cooked through, 10 to12 minutes. Carefully rip open pouches (scalding steam will be released) and serve.