Preheat the oven to 350°F.
Place the potatoes in a roasting pan. Drizzle with olive oil and lemon juice, then sprinkle on salt and pepper. Use a spatula to toss the potatoes until they are well coated.
Whisk together in a small bowl the mustard, olive oil, lemon juice, thyme, rosemary, garlic, salt, and pepper. Set aside.
Using a sharp knife, score the fat layer on the lamb, making cuts no more than 1/2-inch thick and about an inch apart.
Using your hands, smear the mustard mixture all over the lamb, making sure to get it into all the cuts and the crevices on the bottom side.
Using thin twine, tie up the leg so the meat is a snug package. Place the lamb on top of the potatoes, fatty side up. Roast for 1 ½-2 hours until the internal temperature reaches 135°F for medium doneness or 145°F for medium-well doneness.
Let the roast rest for 15-20 minutes before you cut into it. Serve warm with the potatoes.