Melt half of the butter in a large, high-sided frying pan on medium heat. Add leek and cook for about five minutes, until soft. Add the ham and cook for a few more minutes. Now pour into the milk, reduce heat to medium-low and simmer for 10 minutes.
Drain, reserving the leek and ham in one bowl and the milk in another.
Wipe the pan then return to medium heat. Add the remaining butter and once it has melted, add the flour and cook, stirring all the time until you have a thick-ish paste. Pour in about half a cup of the reserved, warm milk and stir until smooth. Gradually add in the remaining milk, stirring as you go, until you have a lovely smooth sauce. Remove from heat, stir in the leek, ham, thyme leaves and lemon zest. Transfer to a bowl, cover and pop in the fridge for a few hours or overnight, until completely cool.
Line a baking tray with paper. Spoon a tablespoon of the now cool mixture into your hand and shape into an oval, place on the baking tray and repeat with remaining mixture. Place this in the fridge for another hour. This helps them keep their shape and makes them much easier to work with.
For the pickled onions, place the sliced onions in a jar and the cider, 1 cup water, the salt and sugar in a small saucepan on medium-high heat. Bring to the boil, stirring to dissolve salt and sugar and simmer for about 5 minutes. Pour this mixture into the jar of onions, seal and let sit at room temperature for at least an hour before serving or keep in the fridge for up to two weeks.
Pour the flour into one bowl, the egg in another and the panko crumbs in a third. Roll one of the balls/ovals in the flour, then the egg then the crumbs and replace to the tray, repeat with remaining croquettas. You can freeze or refrigerate them at this point or continue frying. I prefer to pop them back in the fridge and do a bit of a clean up. Then, when you want to fry and serve, it’s all much less messy!
When ready to cook your croquettas, pour oil into a large saucepan and place on high heat. If you have a thermometer, wait till the temperature hits 180C or throw in a small cube of bread and if it instantly starts bubbling up to the surface, you are ready to go.
Cook in batches, about three or four at a time, about 4 minutes each, or until golden. Transfer to a plate lined with kitchen paper for a minute to drain excess oil, then continue with the rest.
Pile up on a serving platter and serve with the olives, onions and Fresh Fodder Taramosalata.