Heat one tablespoon of the olive oil in a pan, add the onion and garlic and fry for 3-4 minutes until softened.
Add the water and hot fish stock and boil for three minutes.
Add the chopped haddock and cook for three minutes.
Add the cream and three tablespoons of the chervil and season, to taste, with salt and freshly ground black pepper. Simmer for 1-2 minutes, then pour the mixture into a gratin dish.
Place the sweet potato slices on top of the fish sauce mixture, drizzle with the remaining olive oil, and season, to taste, with salt and freshly ground black pepper. Place under a medium grill for 4-5 minutes, or until the top is crisp and golden-brown.
Serve the fish pie at the table in the gratin dish, and garnish with the remaining chopped chervil.