Ask the butcher to remove the chine bone (back bone) from each rack of lamb, to clean and "french" the ribs, and trim the fat cap on the meat.
Preheat the oven to 450 degrees F.
In a small bowl, mix the olive oil with the orange juice, zest, and half of the rosemary to flavor the oil. Pour the rosemary oil over the lamb and rub it into the meat really well; season with a generous amount of salt and pepper. Place the racks of lamb side by side in a large roasting pan, with the rib bones facing up; wrap the bones in foil to prevent blackening in the oven. Toss the radishes with a little oil and put them in the roasting pan next to the lamb.
Roast until the internal temperature of the meat at the thickest point registers 130 degrees F for medium-rare doneness, about 40 to 50 minutes. The radishes should be tender and golden.
Remove the racks of lamb to a carving board to rest, tent them with foil. Drain the excess fat from the roasting pan and set the pan on two burners over medium-high heat. Sprinkle the flour into the pan drippings and stir to cook out the starchy taste of the flour. Gradually pour in the wine, stirring constantly to scrape up the brown bits and avoid lumps. Toss in the remaining rosemary and simmer for 15 minutes to reduce, season with salt and pepper. Add the olives and cook another 2 minutes to heat through.
To serve: Cut the racks of lamb between the ribs into chops. Arrange 2 lamb chops on each plate with the roasted radishes, the olive-wine sauce, and a small pile of watercress.
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