Preheat the oven to 170°C / 325°F / Gas Mark 3.
Place the cream in a sauce pan and add the peppercorns, thyme, rosemary and garlic, then place over a gentle heat. Crumble one third of the cheese into the cream so it can melt more easily. Bring to a simmer, stirring occasionally, and then reduce the heat allowing it to warm and infuse. Remove from the heat after 10 minutes and set aside.
Rub the inside of an oven proof baking dish or gratin dish (measuring about 18cm by 30cm) with the butter making sure it is evenly covered. Slice the potatoes and the turnips very thinly.
Reheat the infused cream and stir thoroughly to ensure the cheese has all melted and pass the mix through a sieve to remove the herbs and peppercorns. Divide the cream between two medium sized mixing bowls. Place the sliced turnips in one bowl and the potatoes in the other and toss gently to coat.
Arrange a layer of potatoes on the bottom of the gratin dish so each slice overlaps the last until it is covered. Now repeat with a layer of turnips and season lightly with a little salt and fresh ground black pepper. Continue like this until you have used all the turnips and potatoes, finishing the top layer with potatoes. As you go, scatter some of the reserved blue cheese every three or four layers you build, spooning a little of the cream mix across the layer as well.
When you have filled the dish, pour any left over cream over the top of the dish and gently tap the whole dish down on the table to help it all settle.
Place the dish on an oven proof tray. Cover with a well fitting piece of parchment paper then bake in the oven. Every 20 minutes or so, remove from the oven and press down gently but firmly with the bottom of a food tub or similar to compress the layers together as they bake. To check it is cooked insert a skewer or small knife, the layers of potato and turnip should offer no resistance.
When done remove from the oven and stand for 20 minutes before serving.
Alternatively, this dish can then be stored in the fridge for one day, even two if well covered with cling film. To serve, peel off the parchment paper and reheat in the oven at 180°C / 350°F / Gas Mark 4 until hot all the way through then serve at once.
