In a saucepan over a medium heat, bring the milk, studded onion, bay leaf and peppercorns to a simmer.
Add the salmon to the pan, and bring back up to a simmer and poach the salmon for 10 minutes.
Remove the salmon from the milk and set aside, and strain the milk into a bowl.
In a separate saucepan melt half the butter over a medium heat, then stir in the flour and cook for a few minutes until light golden-brown.
Whisk in the reserved milk and bring to a simmer, whisking all the time until the sauce has thickened.
Whisk in the double cream and parsley.
Taste and adjust the seasoning with salt, black pepper and lemon juice.
Heat a frying pan until medium hot add the last of the butter and the leeks and sweat for 4-5 minutes until just softened.
Season with salt and black pepper.
To serve, spoon the diced leeks into the centre of the plate. Top with a piece of poached salmon then spoon the parsley sauce over. Halve the remaining lemons and wrap in clean muslin, then tie with a piece of string and serve alongside.