"This is a robust warming gratin made with one of my favourite winter vegetables, the cheap and cheerful swede turnip" says Rory O'Connell.

  1. Preheat the oven to 180c / 350f / gas 4.
  2. Bring a large saucepan of water to the boil and season with a good pinch of salt.
  3. Drop in the sliced turnips, bring back to the boil and simmer for 5 minutes. The turnips will have tenderized slightly but will not be fully cooked. Strain out the turnips, reserving the water for cooking the potatoes. Place the turnips on a tray lined with a tea towel.
  4. Bring the water back to the boil and add the sliced potatoes. Bring to a simmer and cook for 1 minute only. Strain and rinse under the cold tap and place on a tray lined with a tea towel like the turnips. 
  5. Heat the 1 tablespoon of olive oil in a frying pan and add the bacon lardons. Cook stirring until the bacon is crisp and golden. Strain out the bacon and place on a piece of kitchen paper towel to drain. 
  6. To assemble the gratin, grease a 1.5 litre gratin dish with a light smear of butter. Place on a layer of the turnips and potatoes, followed by a sprinkle of thyme leaves, a sprinkle of lardons of bacon and a sprinkle of the grated parmesan.
  7. Season with salt and pepper. Splash on a little of the cream. Repeat the process finishing the gratin with a final sprinkle of the cheese.
  8. Place the gratin in a bain marie in the preheated oven and cook for 60-80 minutes. After 60 minutes, test the gratin with a skewer to see if the potatoes and turnips are tender. The skewer should go through the vegetables with no resistance and the top of the gratin should be a rich golden colour.
  9. The cooked gratin will sit happily in the oven for an hour before serving with the temperature reduced to 50c / 120f / gas ¼ .