For the terrine, place the leeks and spring onions into a wide shallow pan with the vegetable stock and bay leaf. Bring to the boil, then reduce the heat and simmer for 8-10 minutes, or until the vegetables are tender. Remove the vegetables from the pan and set aside.
Return the pan to the heat and boil to reduce the volume of the stock by two thirds.
Meanwhile, soak the gelatine leaves in cold water until softened. Strain, then add to the reduced stock. Whisk to combine, then strain into a bowl. Allow to cool, then season with salt and freshly ground black pepper and stir in the chervil.
Place into the fridge and chill for 30 minutes, or until just setting, but still with a good wobble.
On a work surface, place three large sheets of cling film one on top of the other. Lay the leeks in a row on the cling film, then top with the spring onions.
Spoon over the jelly, then roll the cling film up into a large cigar shape, twisting firmly to enclose at both ends. Place in the fridge for at least an hour until fully set.
For the dressing, whisk the mustard, vinegar and lemon juice together in a bowl. Slowly whisk in the rapeseed oil and season, to taste, with salt and freshly ground black pepper.
To serve, slice the terrine into wedges and place onto serving plates with some slices of ham and a drizzle of dressing.