Preheat the oven at 400°F.

Peel and slice the turnips.

Pour the cream and apple juice in a large saucepan, bring to a boil, lower heat and simmer for about 2 minutes. Add salt, pepper and the mustard.

Put the turnip slices in the juice-cream mixture (stirring to coat all pieces, if needed) and cook for another 5 minutes.

Cut the pancetta in small pieces, and sauté in a non-stick frying pan for a few seconds.

Pour the turnip / cream / apple juice mixture in a baking dish. Sprinkle with the pieces of pancetta.

Bake for about 25-30 minutes, until the gratin is golden on top.