I always recommend making salmon en papillote because it's a delicious and healthy way to cook fish. Plus, the presentation of opening the parchment parcels at the table always impresses dinner guests!
Make the Compound Butter - In a small bowl, combine softened butter, basil, parsley, lemon zest, basil, and salt. Transfer to plastic wrap and twist tightly to create a small butter log. Place it in the freezer while continuing with the other steps. When ready to use, slice into 4 portions.
Heat the Oven - Set the oven rack to the center position. Preheat oven to 450°F (232ºC).
Prepare the Paper Pouches - Cut two 18 x 15-inch pieces of parchment paper. Fold the long side in half and cut it into half of a heart shape. The parchment paper should be large enough to contain one portion of fish and vegetables folded in half. Open up the paper, and brush the inside with vegetable oil.
Assemble the Salmon and Vegetables - Evenly portion the sliced onion, carrots, and asparagus spears on half of each piece of oiled parchment paper. Place one piece of salmon on top of the bed of vegetables. Generously season with salt and pepper.
Add Seasonings and Compound Butter - Top each portion of fish with sliced garlic, ½ teaspoon of lemon zest, and 1 teaspoon sliced green onions. Add 1 slice of compound butter on top.
Seal the Paper Pouches - Fold the other half of the parchment paper over the fish and vegetables. Starting at one corner, tightly crimp the edges to seal it tightly.
Bake - Place the pouches on a rimmed baking sheet. Bake for 8 to 10 minutes. The parchment paper should puff up and brown slightly.
To Serve - Remove from the oven and serve immediately. Transfer to a plate. Carefully cut open and serve with lemon wedges and any remaining compound butter.
Notes
Butter: The compound butter can be refrigerated and made 5 days in advance.
Meal Prep: The pouches can be assembled 1 day in advance.
Foil: If you don't have parchment paper, you can make pouches with foil.