Dice the fish, onions and leeks, chop the garlic and sauté on low heat in butter. Once vegetables are softened add the flour and mix well. Warm up the milk with the fish cube in and make sure cube is well diluted, than mix in with the fish mixture, add the basil, season well and remove from heat. In a clean bowl mix well the ricotta, half the cream, olive dip and half the cheese. In a baking dish, pour in some cream at the bottom and place the lasagna sheets on. Spread the fish sauce, cover with lasagna sheets than with olive ricotta. Repeat this than finish the top with cream and grated cheese. Bake at 180 degrees Celsius for 25 minutes.