Mix the olive oil, mustard, honey, lemon juice and oregano to make the marinade.
Add the potatoes, thyme, rosemary, 5-6 garlic cloves and half of the marinade to a tray.
Place the lamb on top of the potatoes. Make 6-7 slits with a pointy knife and place some garlic gloves.
Cover both sides of the lamb with the remaining marinade and season with salt and pepper.
Cover with parchment paper and foil and bake for 2-2.5 hours. Uncover and bake for 1 more hour.
Serve with some tzatziki and a Greek salad on the side.