This rich side dish is similar to potato dauphinoise - turnips and potatoes are layered up with double cream and garlic and baked
Ingredients
   600g potatoes, such as Maris Piper, peeled
    600g turnips, peeled
    600ml pot double cream
    3 garlic cloves, grated
    ground white pepper, to taste
    large knob of butter, for greasing
Method
    Thinly slice the potatoes and turnips to about 2mm thick – you can use a Japanese mandolin if you have one – then toss together in a large bowl. Pour the cream in a pan, then add the garlic, plenty of salt and some white pepper – be generous when seasoning the cream as the potatoes and turnips aren’t seasoned separately. Bring the cream to the boil, then remove from the heat and put to one side. Grease a large gratin dish and layer a third of the potatoes and turnips in the dish, slightly overlapping each other as if you are building a wall. Pour over a third of the warm cream, then continue in this way with the remaining potatoes, turnips and cream.
    Put the gratin in the same oven as the oxtail and cook for 20 mins. Remove the dish from the oven and push the potatoes and turnips down with a fish slice. This will help to release the starches and the cream will eventually reduce and thicken. Put the gratin back in the oven and repeat the process every 10 mins until the potatoes and turnips feel tight and firm. Return to the oven for another 20 mins and cook until golden brown and tender – test by inserting a sharp knife through the potatoes and turnips. Remove from the oven and leave to cool slightly.
Source: Good Food