1. In a mixing bowl, combine porcini sea salt seasoning, sugar, black pepper, rosemary, and chilli flakes. Pierce lamb with a fork several times on all sides. Brush with vinegar then coat with the rub. Drizzle with any remaining vinegar. Let stand for 1 hour before cooking to absorb flavours of the rub.
2. Preheat a heavy-bottomed skillet over medium heat; add extra virgin oil. Immediately place lamb in pan; cook on all sides until rub has browned.
3. Transfer to a baking sheet; finish cooking at 200°C to desired taste (rare/medium rare is best), turning only once.