Put the lentils in a saucepan, and add equal quantities of fish stock and water just to cover. Allow to simmer for about 20 minutes until the lentils are almost tender and all the liquid has been absorbed. Meanwhile, soften the onion, without colouring, in 2–3 oz / 60–85 g of the butter in a wide shallow pan. Add the fish fillets, and then pour in the remainder of the fish stock and half the cider. Add just enough water so that the fish is nearly covered with liquid. Bring to the boil, and simmer gently for about 4 minutes, until cooked. Remove the fish from the pan, and keep hot. Add the remaining cider to the liquid in the pan, and boil to reduce to 4 fl oz / 110 ml. Cut the remaining butter into small cubes, and whisk it into the liquid, a few cubes at a time, taking care not to boil. Season to taste with salt, pepper and a touch of lemon juice. Divide the lentils between four plates, and place the fish fillets on top. Coat the fish with the sauce and serve.