First make your sauce. Blitz all the sauce ingredients together in a food processor or blender until pale green. Set aside for later.
Slice the leeks into 3/4 inch thick pieces, then rinse and drain in a colander. Pour half the olive oil in a medium pan and cook the shallots and leeks on a medium heat for 15 minutes, or until they have softened. Place the mixture in a large bowl, add the chilli, parsley, spices, sugar and salt, then leave to cool.
Whisk the egg white to soft peaks in a medium bowl, then gently fold into the cooled leek mixture.
In another bowl combine the flour, baking powder, egg, milk and butter to form a batter. Spoon in the vegetable mixture and gently stir to combine.
Heat 2 tbsp of oil in a large frying pan on a medium flame. Once hot spoon in half the batter to form four large fritters. Fry for 2-3 minutes on each side, or until golden and crisp. Remove, drain on kitchen towels and keep warm. Repeat with the remaining batter.
Serve the fritters warm with the sauce and a side of rocket (or your preferred salad leaves). Any leftover fritters will be delicious eaten cold the next day.