Pat fillets dry and sprinkle with lemon juice; roll up, skin side in, and fasten with toothpicks.
In shallow flameproof baking dish over medium-high heat, melt butter; cook onion for 1 minute.
Add clam juice, 1 tbsp (15 mL) lemon juice, bay leaf, tarragon and peppercorns and bring to boil.
Remove from heat and add fillets.
Cover with sheet of greased waxed paper and bake in 350°F (180°C) oven just until fish flakes with fork, 6 to 8 minutes, depending on size of fillets.
With slotted spoon, transfer fillets to warmed serving plate and garnish with lemon slices and parsley. (Strain and reserve cooking liquid for fish stews and chowders.May be frozen.)
Remove toothpicks from fish.