1. Preheat the oven to 425°F.
2. Invert a small bowl and place it in the bottom of a large bowl. Working with one ear at a time, stand the corn on the small bowl and, using a knife, cut off the kernels so that they fall into the large bowl. Remove the small bowl. You should have about 2 cups fresh kernels. (Feel free to use thawed frozen corn, too, if that’s easier.) Add the mushrooms, spring onions, sliced lemon, and a generous pinch each of salt and pepper. Toss to mix the vegetables.
3. Season the fish on both sides with salt and pepper. Lay a 3-foot-long piece of parchment paper on a large rimmed baking sheet. Mound half the corn-and-mushroom mixture on one half of the parchment paper and top with the halibut fillets. Mound the remaining corn-and-mushroom mixture on the fish. Drizzle the olive oil and vinegar on top and then scatter the butter over everything. Fold the parchment over the fish and then, starting at one end, fold over the edge and crimp it to seal. Continue folding over and crimping the edge at 1-inch intervals until you reach the opposite end, then twist the corner to make it airtight. Bake the packet for 20 minutes, until slightly puffed.
4. Using scissors, snip open the packet to release the steam (be careful - it’s hot). Using a spoon, transfer the vegetables, halibut, and any juices to a deep platter or four shallow bowls. Scatter the basil and jalapeno evenly on top and serve hot.
If you can’t find spring onions, have no fear! You can still make this fantastic recipe. Substitute 8 scallions that have large bulbs on the bottom. If using scallions, there’s no need to quarter the bulbs.