Preheat oven to 425°.
Combine first 5 ingredients in a small skillet over medium heat; cook for 1 minute or until fragrant. Transfer toasted spices to a spice grinder or coffee grinder; grind until fine.
Cut 4 (12-inch) squares of parchment paper. Place 1 fillet in center of each square. Sprinkle evenly with salt and spice mixture; top fillets evenly with leeks. Fold paper; seal edges with narrow folds. Place packets on a baking sheet. Bake at 425° for 20 minutes or until puffy and lightly browned. Place on plates; cut open. Serve immediately.