Preheat the oven to 180C/350F. Peel and cut the sweet potatoes, boil with 1 bay leaf and a clove of garlic until tender. Discard the bay leaf and mash the sweet potatoes with the garlic clove until no lumps remain, stir in the butter and hot milk. Season with salt, pepper and nutmeg. Then fold in beaten eggs.
To make the topping mix the breadcrumbs with 4 tablespoons of melted butter and roughly chopped hazelnuts.
Add the mashed sweet potatoes to a greased casserole dish, level the top. Then sprinkle the breadcrumb topping all over. Bake uncovered in the preheated oven for 25-30 minutes.
A few minutes before the casserole is done, fry the sage leaves in the remaining 2 tablespoons of butter. Watch the sage leaves carefully as they burn quickly. Remove from the butter as soon as they start to curl. Scatter the fried sage and sprinkle the flavoured melted butter all over the casserole before serving.