Position an oven rack in the middle and preheat your oven to 400°F (200°C).
Cut four large sheets of parchment paper, approximately 12x16 inches each. Distribute the sliced mushrooms and fresh dill evenly across the center of each parchment sheet.
Place one cod fillet on top of the mushroom and dill mixture on each parchment sheet. Season the fish evenly with salt, black pepper, garlic powder, and onion powder.
Top each fillet with a few thin slices of lemon and a tablespoon of unsalted butter.
Carefully pour ⅛ cup of vegetable broth over the contents of each packet.
Fold the parchment paper over the fish and vegetables, then crimp the edges tightly to create a sealed packet. Ensure no steam can escape.
Arrange the sealed packets on a baking sheet and transfer to the preheated oven. Bake for 20-25 minutes, or until the fish is opaque and flakes easily with a fork.
Carefully remove the baking sheet from the oven. Allow the packets to rest for a few minutes before serving, as the steam inside will be very hot.
Present the packets directly at the table for an impressive reveal, or gently transfer the contents to individual plates. Enjoy immediately.