How to Make It
Heat oven to 425° F. Place the potatoes in a single layer on 4 11-by-15-inch pieces of parchment, dividing evenly. Top with the fish, peas, oil, 1 teaspoon salt, and 1/2 teaspoon pepper. Fold the parchment over the fish, then seal each packet by folding the edges over twice.
Transfer the packets to a baking sheet. Bake until the fish is opaque throughout, 12 to 14 minutes.
Transfer the packets to plates and use scissors to make a small X in the top of each packet. Tear open carefully (they will probably release steam) and top with the pesto.