Preheat oven to 400 degrees. Cut 2 12-by-17-inch pieces of unbleached parchment paper. Fold each in half crosswise to make a crease, then unfold and lay flat.
Mix mushrooms, carrots and leeks with olive oil and season with salt & pepper.
Divide veggies evenly among the parchment paper and top each with piece of salmon.
Add 1 Tbsp. water to each serving. Fold parchment paper over ingredients to create a half-moon shape, making sure all sides are sealed.
Bake both parchment packets on one rimmed baking sheet for ~10 minutes. Serve immediately.