1. Preheat oven to 450 degrees F.
2. Tear off four large sheets of heavy-duty aluminum foil and lay each piece on your workspace.
3. Divide the leeks, carrots, snow peas, and spinach into four equal portions and place a portion onto each sheet of foil. Top each mound of vegetables with a piece of salmon. Sprinkle with ginger, vinegar, and sesame oil.
4. Fold the foil to form a packet around each piece of fish, leaving enough room for heat to circulate. Roll the edges to seal tightly. Place the packets on a cookie sheet and bake until the fish is cooked (10 minutes per inch of fish thickness).
5. Sprinkle with lemon juice and garnish with lemon to serve.