Preheat oven to 180°C. Place cream, brandy, garlic and half the rosemary in a small saucepan over medium heat. Bring to a simmer, then set aside for 20 minutes to infuse.
Strain cream into a bowl, discarding solids, and whisk in the Dijon mustard.
Roughly chop remaining rosemary and combine with parmesan in a bowl. Set aside.
Combine parsnip and potato in a large bowl. Season with salt flakes and freshly ground black pepper.
Pour a little infused cream into a 23cm x 5cm-deep (1.5L capacity) ovenproof frypan. Cover base with one-third of the parsnip and potato, then scatter with one-third of the parmesan mixture and half the onion. Pour over one-third of the remaining cream mixture. Repeat layering once, then finish with a layer of parsnip and potato, remaining parmesan mixture and remaining cream (the layers don’t have to be perfect, so don’t worry too much about the order).
Cover with a layer of baking paper, then foil and bake for 45-50 minutes until parsnip is tender.
Remove foil and baking paper. Bake, uncovered, for 20 minutes or until golden and bubbling (increase temperature to 200°C if you need a little more colour in the last 5 minutes). Rest for 10 minutes, then top with the truffle cheese and scatter with extra rosemary. Serve immediately.
