Melt the butter in a saucepan over a low/medium heat and gently saute the leeks and garlic. Add a pinch of salt to the ban, this will stop the leeks browning and they will soften nicely.
Cover the base of a 9" gratin/pie dish with a layer of potatoes (use half of the potatoes at this stage), followed by a layer of fennel and a layer of the leek mixture, taking care to lightly season each individual layer.
Finish the dish with a final layer of potato which should be seasoned with salt and pepper. Pour over the cream, cover the top of the gratin.
In a separate bowl mix in the parmesan cheese with the breadcrumbs and sprinkle over the dish in a thick layer.
Bake the gratin in an oven at 190C/170CFan for 1 hour or until the potatoes are cooked through.
This basic gratin recipe is easily adapted for other vegetables, whatever you might have to hand. Onions can be used in place of leeks if preferred.
If you want to cut back on the the cream, simply substitute half the cream for milk.
We like to serve this dish with fish, chicken or pork, all of which go very well with the aniseed flavour provided by the fennel. Due to the rich gratin sauce we would recommend cooking your meat or fish simply.