Method

To make the kale crisps, pre-heat the oven to 130C/ 260F/gas mark ½ and line a large baking tray with greaseproof paper.
Remove any woody stalks from the kale, rinse well and pat dry. Place the kale in a large bowl and toss with a drizzle of olive oil. Add the ground cumin, salt, and toss again, then spread the kale out on the baking tray and bake for 20 minutes, or until the kale is crisp and still green. Once cooked, remove from the oven and allow to cool. Turn up the heat to 200C/ 400F/gas mark 6.
Peel the vegetables and leave whole. Place the potatoes in a large saucepan and cover with cold water, then bring to the boil and add the parsnips and sweet potatoes. Reduce the heat and simmer for 20 minutes, or until you can cut through them with a knife. Drain the vegetables and allow to air dry in the colander until cool enough to handle.
Butter a 30 cm x 25 cm baking dish, scatter the sliced cloves of garlic around the buttered dish and season with salt and pepper. In a large bowl, whisk the double cream and egg yolks together, stir in the grated cheese and chives, then pour into a large jug.
Once the vegetables are cool enough, cut them into 1cm slices. Pour ¼ of the sauce into the base of the dish and scatter over a few tomatoes, thyme leaves and a layer of vegetables. Repeat twice more and finish with a layer of sauce on top.
Place the gratin in the oven and bake for 20 minutes or until golden and bubbling. When ready, remove from the oven and finish by scattering over the crispy kale.