Fish is so good for you, and so is steaming, a great method of cooking because none of the flavors or nutrients are lost. This sauce tastes very creamy and rich without having any of the fat from cream.
Remove the seeds from the vanilla pod and place both the seeds and pod on a plate with the lemon juice and olive oil. Mix up with a fork, then place the fish on the plate and turn it so both fillets are covered. Place the vanilla seeds and lemon leaf on top of the fillets, reserving the vanilla pod.
Split the leeks down the middle, 1-inch from the end, and wash under cold water. Mix the peas, broad beans and new potatoes together.
Put the milk into a saucepan with the reserved vanilla pod and slowly simmer over a medium heat. In a bamboo steamer* place the leeks on the bottom, then cover with the pea mix. Sprinkle with salt and freshly ground black pepper, then place the steamer over the pan of milk. In a second bamboo steamer place the lemon leaf, and the fillets skin side up. Open up the score marks and pour over the juices from the plate. Place the lid on top and cook for 10 to 15 minutes on a slow simmer.
Remove the steamers from the pan, place some vegetables on each plate, and top with the fish. To finish the sauce, remove the vanilla pod from the milk and whiz up with a cappuccino frother or 'whizzer', or in a liquidizer. This gives the sauce a thick cappuccino effect, which is both visually and texturally very nice.
*If you do not have bamboo steamers, use a colander or a normal steamer.