1. Preheat oven to 400°. Place 4 (12- x 7-inch) rectangular pieces of parchment paper on a baking sheet.
2. Divide half of fennel and red onion evenly among parchment pieces. Top fennel and red onion with fish, remaining fennel and red onion, 2 lemon slices, 2 tablespoons sauce, and 2 tarragon sprigs; fold up, and crimp parchment paper to form a tight pouch.
3. Bake fish, in middle of oven, until cooked through (about 15 minutes). Place parchment packets on serving plates; open carefully to release steam. Serve fish alongside the couscous.