Pat the lamb pieces dry. Rub with half a lemon and season with salt and pepper.
In a large pan with a tight-fitting lid, heat the oil and butter over medium heat until foaming, then add the lamb and ham. Cook, stirring occasionally, until the lamb pieces begin to brown, then add the onion, and continue to cook until the lamb is nicely browned all over.
Sprinkle over the flour and stir to coat all the lamb. Lower the heat and add the wine, then cover, and simmer until the lamb pieces are fork tender, about 30 minutes.
In a small bowl, beat together the yolks, lemon zest and juice, and the marjoram. When the lamb is ready, slowly add the yolks to the pan, whisking constantly. The sauce should be thickened and smooth. Garnish with the parsley and serve right away.