Preheat the oven to 375*F. Place the cauliflower in an ovenproof pan and drizzle with the olive oil, then sprinkle with salt. Roast for 20 minutes, or until the cauliflower has browned to a deep nutty color on top.
While the cauliflower is in the oven, chiffonade 4 sage leaves (ie slice into thin ribbons). Melt the butter in a saucepan over medium heat and add the sage and garlic. Cook until the garlic is fragrant, about 1 minute, then add the cream and stir to combine. Add the nutmeg and take the cream off the heat, allowing to to steep.
Remove the cauliflower head from the oven and turn the head up to 400*F.
Carefully, because it is still hot, slice the cauliflower into thin “steaks”. Layer an 8″x8″ baking pan with parchment paper or tin foil and arrange the cauliflower in one layer. Scatter half of the leftover sage leaves on top, then add another layer of sliced cauliflower. Top this with the rest of the sage leaves.
Pour the cream mixture evenly on top, making sure the cream gets down into any crevices. Top evenly with the parmesan cheese, then the breadcrumbs, and slide back into the oven for 18 minutes.
After the au gratin has baked, broil for a further 2 minutes, or until the breadcrumbs have browed fully. Remove from the oven and serve hot.