Preheat the oven 175°C fan.
Begin by sauteing the onion in 15g of the butter until really soft and translucent, cool.
Place the cold mashed potato, breadcrumbs and egg in a bowl and set to one side for 5 minutes.
Now cut the fish into small pieces and pulse in a food processor, don’t over blend.
Place the blended fish into the breadcrumb mix, add the cooled onions, lemon zest then season with salt and white pepper.
With damp hands take a dessertspoonful of the mixture and shape into a ball. Repeat with the remainder. Now place the fishballs on a plate or tray, cover and pop in the fridge for an hour or until you need them.
Place the fishballs in a hot frying pan with the remaining butter and sunflower oil. Turn so they are golden brown on all sides. Put the fishballs on a baking tray and place in the oven to continue to cook for 8-10 minutes. Now you make the sauce.
To make the mushroom sauce: clean the mushrooms with a brush. Tear or cut in half. Sauté in the butter until all the liquid from the mushrooms has evaporated. Season with salt and pepper. Now add the chopped shallot and garlic, sauté on a low heat until soft. Now add the cream, soy sauce and stock. Bring to the boil and simmer for 2-3 minutes. If you’d like to thicken the sauce slightly, mix the cornflour with 2 tbls of cold water and stir into the mushroom sauce and simmer for a couple of minutes. Check the seasoning.
To make the Fennel Sauce: place the sliced fennel in a hot frying pan with the butter and sunflower oil. Stir and fry until the fennel starts to take on some colour. Now add the white wine, reduce by half, then add the stock and cream. Simmer for a couple of minutes. If you’d like to thicken the sauce slightly, mix the cornflour with 2 tbls of cold water and stir into the sauce and simmer for a couple of minutes. Check the seasoning.
Serve with a salad or steamed vegetables.