Preheat the oven to 400°F. Cut four pieces of parchment paper each 18 in. long
Place a fish fillet on the center of a piece of parchment. Season with a small pinch each of salt and pepper, then drizzle with olive oil.
Place three lemon slices on each fillet, overlapping them slightly to cover the fish. Sprinkle one-fourth each of the corn, asparagus, tomatoes (if using) evenly around the fish, then drizzle with a little olive oil and season again with a small pinch each of salt and pepper.
Bring the long sides of the paper together, and fold the top edges down together to create a 1-in seal, then continue to fold down tightly over the fish and vegetables. Twist the open ends of the parchment in opposite directions to prevent steam from escaping.
Repeat the process with the remaining ingredients and parchment. Place the packets on a baking sheet. (If not baking immediately, refrigerate for up to 4 hours.)
Bake until the packets are lightly browned and have puffed up, about 15 minutes. Transfer each packet to a plate and let stand for 5 minutes. Using sharp scissors, cut an X into the center of each packet and carefully pull back the parchment and sprinkle with herbs. Serve immediately.