How to Make It
Preheat the oven to 450°. In a small bowl, mix the dry mustard with 3/4 teaspoon each of the salt and pepper. Season the lamb chops with the mustard mixture and the marjoram. Drizzle 2 tablespoons of the olive oil over the chops and let stand at room temperature for 15 minutes.
Meanwhile, in a medium saucepan, heat the remaining 3 tablespoons of olive oil. Add the celery and leeks and season with salt and pepper. Cover and cook over moderate heat, stirring occasionally, until softened but not browned, about 5 minutes. Add the beans and the Dijon mustard and stir gently until heated through, about 2 minutes. Cover and keep warm.
In a large, heavy, ovenproof skillet, heat the oil. Add the lamb chops and cook over high heat, until well-browned, about 1 minute per side. Sear the fatty edge of the chops for 20 seconds.
Transfer the skillet to the oven and roast the chops for 8 minutes for medium rare, turning them once. Arrange the chops on a platter and let rest for 5 minutes. Spoon the beans alongside and serve.