Preheat the oven to 400ºF and grease a baking dish.
Peel parsnips and beets, thinly slice (a mandolin does an excellent job) and layer 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. Grate in the orange zest, if using. Sprinkle with 1 1/2 tsp salt and a pinch of black pepper.
Pour the cream over the vegetables, pressing them to submerge in the liquid and place the rosemary sprigs on top. Cover with tin foil and bake for 40 to 45 minutes, until the vegetables are almost tender.
Remove the foil and bake for 20 minutes longer, or until set on top and browning at the edges. Cool for five minutes before serving.