Pour milk & Cream into a saucepan and add thyme/rosemary and some black pepper. Bring to just under the boil and then remove from heat. Leave aside for around half an hour. Strain through a fine sieve and discard herbs.
Slice peeled turnips and potatoes into rounds, not too thin and par boil for about 4 mins. Pat dry.
Butter a casserole dish and arrange the vegetable slices in an overlapping fashion in the baking dish, alternating them and seasoning with salt and pepper as you go to taste. Dot with butter, if using. Pour cream over top (it should almost cover the vegetables). More cream can be added if necessary. Pat down and sprinkle grated cheese on top.
Bake in oven at 180C (160C Fan assisted oven) until vegetables are tender, about 40 minutes.
350g Rooster potatoes
350ml double cream
6 sprigs thyme/rosemary, or mixture of both
100g gruyere cheese