Preheat the oven to 400°. Cut individual squares of the parchment, one per fillet, large enough to wrap and enclose the pieces completely. Place each piece of fish on its wrapper. Distribute the leeks, red pepper, parsley or cilantro, and mushrooms if desired, equally over the fillets. Mix the lime juice with the mustard, curry or jerk, and Crystal or Tabasco, to make a dressing, then drizzle this evenly over the fish. Add salt and pepper.
Now enclose each dressed fillet in its wrapper. Some folks make an art of this, for presentation, but the key point is to tightly fold over the edges of the paper so the contents are well sealed. Lay the packets side by side in a baking dish and bake for 15 minutes. Serve immediately, either in the parchment, or removed to a platter.