Fennel and potato gratin

1) Preheat the oven to 200C/400F/Gas 6 (180 fan).
2) Bring a large pan of water to the boil and add the fennel, onions and potatoes. Turn down to a simmer and cook for about 5 minutes.
3) Drain the vegetables and place in a greased oven proof dish.
4) In a frying pan put the garlic and butter and heat until the butter has melted.
5) Pour the garlic butter over the vegetables, season to taste and mix gently to coat.
6) Sprinkle with the Parmesan.
7) Bake for 35-40 minutes.