Heat oven to 400°.
Combine corn, tomato, basil, and garlic in a medium bowl. Season with salt and pepper.
Cut 2 pieces of 16-inch-long parchment paper. Fold each in half lengthwise and unfold. Divide corn-tomato mixture evenly between parchment, arranging along one side of the fold. Top with fish and season with salt and pepper. Squeeze lemon over fish and top with 2 pieces of butter.
Fold parchment over fish leaving space for steam to circulate. Seal by making small, overlapping folds around edges. Transfer to a baking sheet and bake for 10–12 minutes, packets will puff up.
To serve, immediately transfer packet to a serving dish and cut open releasing steam. Sprinkle with basil and lemon zest. Serve with bread to sop up juices.
NOTE: I highly recommend purchasing a nut milk bag, it makes the process of homemade mylks much easier. A variety of choices can be found online.