In a bowl, combine the lemon zest, 2 tbsp. EVOO and the garlic; rub on the lamb and season with salt and pepper. Place the lamb on a foil-lined pan. In a bowl, toss the potatoes with 1 tbsp. EVOO and season with salt and pepper; arrange around the lamb in the pan.
Position an oven rack 4 inches from the broiler element and preheat the broiler. Broil the lamb until well browned, 8 to 10 minutes. Flip and continue broiling until cooked to medium-rare (125 degrees on an instant-read thermometer), about 5 minutes; transfer to a cutting board and let rest for 15 minutes. Thinly slice against the grain.
Toss the potatoes with the pan juices; broil until browned, 3 to 5 minutes.
Meanwhile, in a medium bowl, whisk together the mustard, 1 1/2 tbsp. lemon juice and the remaining 1 tbsp. EVOO. Toss the arugula and onion with the dressing; season with salt and pepper. Serve the salad alongside the lamb and potatoes.