It's a super easy, super fast, and really flavorful way to prepare halibut but the best part happens at the table, when you tear open that parchment paper package. You can see the quinoa, the bright vegetables, and the steam rising off to the side...that's when you know you're in business.
Preheat oven to 400 degrees F (convection, if possible). Prepare the vegetables: Peel and slice the turnip on a mandoline, then julienne into matchsticks. Set aside. Do the same for the carrots. Then slice the shallot, jalapeno, sugar snap peas, garlic, and cabbage. The vegetables should be roughly the same size, so they cook evenly.
Assemble the papillote: Lay parchment paper on a rimmed baking sheet. Place 1/2 cup quinoa in the middle of the parchment and then layer 1/4 of each vegetable on top (for 4 individual papillotes). Season with a pinch of salt, pepper, and a drizzle of olive oil. Place a halibut fillet on top and drizzle with more olive oil, salt, and a bit of lemon zest. Bring the two sides of the parchment up over the halibut, align the edges, and fold down the parchment tightly over the fish. Turn the packet and fold the ends of the parchment under the fish to create a tight packet. Bake 8-12 minutes.
Finish the dish: The halibut is cooked when the fish is opaque and firm to the touch. Set the packet on a serving plate and carefully open. Finish with 1 tablespoon olive oil, scallions, and lemon juice and serve.