Heat oil in a pan over low heat. Fry the leek and pork belly for 5 minutes. Remove from heat and let cool for 2-3 minutes.
Put the bread in a bowl. Add the cooled leek and pork belly, ham, and cheese.
Whisk the eggs into the milk with salt, pepper, and a pinch of nutmeg. Pour the milk mixture into the bowl, add the parsley, and mix thoroughly until combined. Let the mixture rest for 30 minutes.
Shape 8 patties from the mixture using slightly damp hands.
Heat the clarified butter in a pan over low heat. Fry the patties until golden brown on both sides.
Remove the patties and let them drain on kitchen paper for 1-2 minutes.