Preheat the oven to 200ºC/400ºF/gas 6 and grease a 1.5 litre baking dish.
Scrub the parsnips and beetroots, thinly slice (you can use a mandolin, but remember to use the guard!) and layer up in the baking dish.
Put the cream, crème fraîche, whole unpeeled garlic and rosemary sprigs in a saucepan, and bring to a simmer. Take off the heat, finely grate in the orange zest and season with 1½ teaspoons of sea salt and a big pinch of black pepper.
Pour the cream over the veg, pressing them to submerge in the liquid and arrange the rosemary sprigs on top. Cover tightly with tin foil and bake for 40 to 45 minutes, or until the veg is almost tender.
Remove the foil and bake for a further 20 to 25 minutes, or until set on top and browning at the edges. Cool for 5 minutes before serving.