Preheat the oven to 450°F.
Cut four sections of foil (or parchment paper), 18" x 12" each.
Spread one of the foil sheets onto a baking sheet and coat with cooking spray.
In the center of the foil section, place ¼ of the onion and drizzle with olive oil. Top with one cod portion, ¼ of the asparagus, ¼ of the tomato, ¾ teaspoon of olive oil and ½ tablespoon of lemon juice. Sprinkle with salt and pepper.
Crimp the edges of the foil together so that all ingredients are sealed inside.
Repeat with the remaining sections of foil to make four pouches total.
Place the pouches on the baking sheet. Bake until the pouches are expanded and puffy, with the fish cooked to opaqueness (about 12 minutes).
Remove the contents of each pouch on a plate and sprinkle with basil.
