Sauté the onion in the butter and once softened add the flour, salt and pepper to form a thick paste. If you can see the bottom of the pan when you move the flour mixture with a spoon, it is ready.
Whisk in the milk until smooth, then fold in the ham, hard boiled eggs, wine and seasoning. Remove from heat and let cool to room temperature before refrigerating for at least 2 hours. The colder it is, the easier it is to mould.
Take a tablespoons full of the mixture and roll into a hot dog shape by rolling it back and forth with your palms. Party size croquettes are about 1cm thick and 4cm long, while the larger ones are about 2cm thick and 7cm long.
Dip the croquettes into a bowl containing whisked eggs and one tablespoon water. Next dip into the breadcrumbs and repeat for all croquettes.