Preheat the oven to 425 degrees F. Mix the seasonings (vinegar, juice, wine, zest, spices, etc.) together in a small bowl; set aside.
Fold 4 (20-inch) parchment paper squares in 1/2 lengthwise and cut out a large heart shape. Unfold and arrange on 2 baking sheets. Cook's Note: Aluminum foil can be substituted for parchment paper.
Evenly divide the vegetables among the 4 heart-shaped parchment packets. Place fish fillet on top of vegetables and season. Top with a small pat of butter.
Fold the top half of the parchment over the fish, overlapping small folds along the edges and sealing. Bake for 10 to 15 minutes, until the parchment paper puffs up.
Transfer the parchment packets to plates. Carefully cut the packets open, avoiding the hot steam and serve.
En Papillote Variations:
Wild Mushrooms en Papillote: Replace the fish fillet with 1 pound mushrooms (equal parts chanterelles, morels, and shiitakes). Add fresh chives, fresh parsley, fresh tarragon, white wine, and increase the butter to 1 tablespoon.
Spring Vegetables en Papillote: Replace the fish fillet with 4 cups vegetables like zucchini, eggplant, asparagus, etc. Season the vegetables with thinly sliced garlic, fresh herbs, fresh lemon juice, zest, and salt and pepper.
Mediterranean Halibut en Papillote: Top the halibut with pitted kalamata olives, sliced red onion, fresh parsley, fresh lemon juice, and fresh oregano. Add white wine for steaming.