Herb-Crusted Rack of Lamb with New Potatoes

Place potatoes in a large saucepan and cover with 1" cold water; season with salt. Bring to a boil, reduce heat, and simmer until tender, 10 –15 minutes. Drain. Meanwhile, preheat oven to 400 °F. Season lamb with salt and pepper. Heat 2 tablespoons oil in a large skillet over medium-high heat. Cook lamb, turning occasionally, until golden brown all over, 6 –8 minutes; transfer to a foil-lined baking sheet, placing fat side up. Mix garlic, parsley, dill, mustard, cumin, and 1 tablespoon oil in a small bowl; season with salt and pepper. Spread herb mixture over lamb and roast until an instant-read thermometer inserted into thickest part of meat registers 130 °F for medium-rare, 18 –22 minutes. Transfer to a cutting board and let lamb rest 5 minutes before cutting into double chops. While lamb is resting, heat remaining 1 tablespoon oil in a medium skillet over medium heat. Add potatoes and watercress and cook, tossing, until watercress is just wilted, about 1 minute. Mix in vinegar and season with salt and pepper. Serve lamb with watercress and potatoes.