The molten filling inside is essentially a savoury beef stew that's been thickened until stiff enough to roll into a ball and coat in breadcrumbs.
A rich veal or beef ragout is thickened with a roux, chilled until firm, then rolled into balls, breaded and deep-fried until the outside is crunchy and the inside stays gooey. Amsterdam's brown cafés serve them by the plateful with mustard for dipping, especially alongside beer.