Preheat oven to 375°F. First roast sweet potatoes whole, until tender. Then slice into halves or quarters depending on size. Score the flesh and sear in a hot pan with Olive Oil until golden brown.
To make the glaze, cook out red onion, garlic, and half the butter over medium heat until sweet. Remove from the pan and reserve. Over high heat, add the rest of the butter and harissa. Cook for a few minutes, then add fish sauce and rice wine vinegar and cook until it is reduced by half. Last, add the honey and garlic and black pepper. Cook until the sauce has turned into a thick glaze (reducing it by approximately half).
begins to caramelize. Top with toasted hazelnuts and lemon zest.