1.Place fish fillets in a small saucepan and just cover with water. Roughly slice half the fennel bulb and add to the pan with the fish stock. Bring to the boil and simmer for 1 minute. Remove from heat and set aside to cool.
2.In a bowl stir together the lemon juice, mustard, crème fraîche, chopped dill and horseradish. Finely chop the remaining half of the fennel bulb and add to the crème fraîche mixture.
3.Carefully flake the smoked fish into the crème fraîche mixture. Drain the cooled fish fillets (discard fennel) and add to bowl. Gently fold together and season to taste with salt and pepper.
4.Transfer to a jar or serving dish and refrigerate until required (the potted kahawai will last up to seven days, covered, in the fridge).
5.Serve with the cornichons and sliced, fresh crusty baguette.