METHOD

1. Pre-heat the oven to 180C/350F.
2. Thinly slice the carrots and parsnips with a knife or mandolin. Set aside.
3. In a large frying pan, cook the leeks in one tablespoon of butter until soft. Remove from the heat and set aside.
4. Place the cauliflower florets in a large saucepan and cook in boiling water until soft. Drain. Place the florets in a high-powered blender with one tablespoon of butter. Season with salt and pepper. Blend until smooth and creamy.
5. To assemble the gratin, spread some cauliflower purée on the bottom of a pie dish. Top with carrot and parsnip slices. Top with some leek and a small sprinkling of cheese. Repeat, finishing with a thick layer of cauliflower purée. Sprinkle with the remaining cheese.
6. Place in the oven and cook for 1 hour or until the carrots and parsnip are al dente. If the top is browning too quickly, cover it with foil.