McDonald's claims the McKroket is a 'real traditional Dutch burger'. It vaguely describes the sandwich as having McKroket mustard sauce and a McKroket patty. But what is the mysterious McKroket? It's actually a croquette with a crunchy breadcrumb coating, filled with a beef ragout and topped with creamy mustard sauce. The burger is a bit of an acquired taste, but it's a hit in the Netherlands.