How to Make It
Preheat the oven to 425°. Season the lamb with salt and pepper. In a very large skillet, heat 1 tablespoon of the olive oil until shimmering. Add the lamb fat side down and cook over moderately high heat until well browned, about 4 minutes. Turn the lamb fat side up and roast in the oven for about 18 minutes, until medium-rare. Transfer the lamb to a work surface and let rest for 5 minutes.
Meanwhile, pour off all but 1 tablespoon of oil from the skillet. Stir in the shallot and cook over moderately low heat, until tender, 1 minute. Whisk in the mustard and vinegar. Gradually whisk in the remaining 6 tablespoons of olive oil. Add the beans to the skillet, season with salt and pepper and stir to combine.
Spread the watercress on a serving platter. Pour the white beans over the watercress. Cut the lamb between the ribs into chops. Arrange the chops on top of the white beans and serve.