Bring the water to the boil and add the lemon juice, spices, herbs and shallots. Cook for 10 minutes to make a vegetable stock.
Place leeks in a frying pan and cover with the vegetable stock. Simmer, covered for 10-15 minutes until leeks are soft. Leave to cool.
Beat the yoghurt, egg yolks and lemon juice together and place over a pan of gently simmering water. Cook, stirring frequently, for about 15 minutes until thickened. Season with salt, pepper and mustard.
Drain the leeks and cut into 2 or 3 diagonal pieces. Arrange in a serving dish, spoon over the yoghurt sauce and sprinkle with parsley.