- Combine butter, miso, chili, and lime zest in a small bowl.
- Fold 4 pieces of 16-inch-long parchment in half lengthwise and unfold. Arrange carrots on each piece of parchment along one side of the crease. Top each with a piece of fish; lightly season with salt, and smear with 2 tablespoons miso butter.
- Fold parchment over fish and seal by making small overlapping folds around edges. Bake for 10–12 minutes until packet puffs up.
- To serve, cut packets and carefully transfer carrots and fish to serving plate, top with scallions, and serve with lime wedge. Or serve fish right out of cut packets.