Heat oven to 425°. Place lamb in a roasting pan and pat dry with paper towels. Using a sharp knife, make incisions in a crisscross pattern across the lamb. Stud with the garlic and sprinkle with the Italian Rub. Drizzle with olive oil and rub well, coating the entire piece of meat.
Roast lamb for 15 minutes, then reduce the heat to 350°. Add the potatoes to the pan, tossing to coat with any pan juices. Continue roasting for 30 minutes, then add tomatoes. Continue to roast until lamb reaches 140°, about 15 minutes more—lamb should still be pink in the middle. If you prefer your lamb to be cooked to medium or medium-well, leave it in the oven for an additional 15 minutes.
Remove from the oven, cover with foil and let rest for 15 minutes before carving.