Preheat your oven to 180°C (350°F).
In a small bowl, mix together minced garlic, olive oil, dried oregano, salt, and black pepper.
Rub the mixture all over the lamb leg, making sure to get into all the nooks and crannies.
Place the lamb in a roasting pan and roast in the preheated oven for about 90 minutes, or until the internal temperature reaches 60°C (140°F) for medium-rare.
While the lamb is roasting, prepare the tzatziki sauce. In a mixing bowl, combine Greek yogurt, grated cucumber, minced garlic, olive oil, lemon juice, chopped dill, and salt. Mix well.
Once the lamb is done, remove it from the oven and let it rest for 15 minutes before carving.
Serve the lamb slices with a generous dollop of tzatziki sauce on the side.