1. Preheat oven to 400°.
2. Cut 4 (18-inch-long) pieces of parchment paper. Spoon corn and zucchini evenly onto center of each piece. Top evenly with asparagus, placing spears lengthwise on paper. Place fillets lengthwise over asparagus; top evenly with lemon slices. Place tomatoes around edges. Drizzle each serving with 1 teaspoon butter; sprinkle evenly with salt and pepper. Bring long sides of paper together; fold down tightly over fish and vegetables. Twist ends to enclose contents. Place packets on a large baking sheet.
3. Bake at 400° for 15 minutes. Place 1 packet on each of 4 plates. Let stand 5 minutes; cut open.