Preheat your oven to 425 degrees. Bring the lamb to room temperature
In a small bowl mix the softened butter, minced garlic, rosemary, parsley, and mustard
Pat the lamb dry with paper towels. Make small slits all over the meat , using a paring knife.
Rub half the garlic butter all over the exterior of the lamb, work the remaining butter mixture into the slits using your fingers.
Toss the potatoes, carrots and onions with the 2 Tbsp. of olive oil in a large roasting pan. salt and pepper to taste.
Place the lamb on top of the vegetables. Roast at 425 degrees for 15 minutes to get a good sear on the meat then lower the heat to 325 and continue roasting. Roast for about 15-20 minutes per pound for medium -rare ( 125 F -130 F internal temperature) or longer if you prefer it more done. A 4.5 Lb. roast typically takes about 1hour 15 minutes to 1 hour and 30 minutes.
Let lamb roast rest 10-15 minutes before slicing
Plate the lamb surround it with the roasted vegetables, pour the pan juices over the top.