Method

• 1.
  Preheat the oven to 190°C.
• 2.
  Melt the butter in a 30cm x 20cm flameproof baking dish over medium heat. Add the onion, garlic and half the thyme, and cook, stirring, for 8 minutes or until the onion is softened. Add the sliced potato and stir well, then pour over the milk. Bring to the boil, then reduce heat to low and simmer, stirring constantly to release the starch from the potato slices, for 3 minutes or until the potato is heated through.
• 3.
  Remove from the heat and stir through the cream and creme fraiche, then fold through the brussels sprouts. Add the lemon zest and half the parmesan. Season, and stir gently to combine, before grating over the remaining 1/2 cup (40g) parmesan.

• 4.
  Place the bread in a food processor and pulse to coarse breadcrumbs. Transfer to a small bowl. Add the chestnuts, oil and remaining thyme, and stir well to combine. Sprinkle the chestnut crumb over the top of the vegetable mixture.
• 5.
  Transfer to the oven and bake for 50-55 minutes or until the vegetables are softened and the top is bubbling and golden.
• 6.
  Remove the gratin from the oven and set aside for a few minutes to cool slightly, then serve with a crisp green salad on the side.