Heat the oven to 200C/fan 180C/gas 6. Put the cream and cider into a large pan and simmer for a couple of minutes. Season then add the mustard and turnip slices and cook for 5 minutes.
Fry the bacon until crisp. Tip the turnip mix and bacon into a gratin dish, leaving some bacon for the top. Cook for 25-30 minutes until golden and bubbling.