Preheat oven to 200˚C.
Melt the butter in a medium-sized pan over a low heat, add the garlic and sauté for 1 minute. Add the leek and ½ the lemon zest to the pan and sweat for 5-6 minutes until wilted.
Pour the cream over the leeks and garlic and warm through until bubbling. Stir in the crab, season to taste and remove from the heat. Pour the creamy leek and crab mixture into a suitable gratin dish.
Mix the breadcrumbs with the sage, remaining lemon zest, pecorino and hazelnuts. Sprinkle the breadcrumb mixture over the creamy leek and crab, drizzle with a little olive oil and bake in the oven for 25 minutes or until the breadcrumbs are golden brown and the cream is bubbling up around the edges.