Preheat your oven to 400°F (200°C).

In a large skillet, heat the olive oil over medium-high heat. Add the sliced red onions and cook until they are charred and caramelized, about 8-10 minutes. Remove from heat and let them cool slightly.

In a small bowl, mix the Dijon mustard, honey, chopped dill, salt, and pepper. Rub this mixture all over the leg of lamb, ensuring it’s evenly coated.

Place the charred red onions and whole garlic cloves in the bottom of a roasting pan. Lay the lamb leg on top of the onions and garlic.

Pour the beef or lamb stock into the pan around the lamb, being careful not to wash off the crust.

Roast in the preheated oven for about 1 hour and 30 minutes, or until the internal temperature reaches 135°F (57°C) for medium-rare. Adjust cooking time according to your desired doneness.

Once cooked, remove the lamb from the oven and let it rest for 15-20 minutes before slicing.

While the lamb is resting, strain the pan juices into a saucepan. If you prefer a thicker gravy, mix the cornstarch with a little water to create a slurry and add it to the pan juices. Bring to a simmer over medium heat, stirring until thickened.

Serve the sliced lamb with the velvety roasted garlic gravy drizzled over the top, and garnish with fresh dill.