Preheat the oven to 350 degrees. Cut away any excess fat from the lamb. Season liberally on all sides with salt and freshly ground black pepper.
Stud the leg of lamb evenly with the garlic cloves, pushing them in far enough so that they don’t get squeezed out as the meat constricts during cooking.
Place the roasting pan on the stove. Pour just enough oil into the pan to cover the bottom. Sear the lamb on all sides.
Brush the Dijon and roasted garlic seasoning evenly over the top of the lamb, then place into the oven. Note: If you are lucky enough to have a convection oven, another searing method that can be a bit less cumbersome for a whole leg of lamb is to preheat the oven to 450 degrees. Roast the lamb, turning halfway, for 15 minutes. Reduce the temperature to 350 degrees and cook for an hour and 20 minutes more, or till an internal temperature of 130 degrees is reached
Roast for 1½ hours or until an instant-read thermometer registers at 130 degrees F. This will give you a nice medium-rare to medium. Remove from the oven and allow to rest in a warm area of the kitchen for at least 10 minutes before carving.