Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Peel the sweet potato and cut it into uniform 1-inch cubes for even roasting.
Mince the garlic cloves and finely chop the fresh rosemary leaves.
In a small bowl, combine the olive oil, minced garlic, chopped rosemary, sea salt, and black pepper.
Rub two-thirds of the herb oil mixture over the lamb leg, ensuring it is well coated on all sides.
Toss the sweet potato cubes with the remaining herb oil mixture and spread them in a single layer on the baking sheet.
Place the lamb on the baking sheet with the potatoes and roast for 20 minutes.
While the lamb roasts, trim the woody ends off the asparagus spears.
Add the asparagus to the baking sheet, tossing lightly with the juices already on the pan.
Roast for an additional 10 to 15 minutes until the lamb reaches an internal temperature of 145°F for medium-rare and the potatoes are golden brown.
Remove from the oven and let the lamb rest for 5 to 10 minutes before slicing against the grain.
Serve the sliced lamb alongside the caramelized sweet potatoes and roasted asparagus.