Preheat the oven to 200°C (fan).
Cut the butternut squash in half, scoop out the seeds, peel, and slice thinly using a mandolin or sharp knife.
Mix the single cream with salt and grated nutmeg.
Grease a 20cm x 20cm rectangular baking dish with olive oil. Layer any irregular or smaller pieces of butternut squash at the bottom, sprinkle half of the grated Gruyère over the squash, and pour 1/3 of the cream mixture evenly over the top.
Neatly arrange the remaining butternut squash slices on top, sliding the garlic slices and sage leaves in between.
Pour the remaining cream mixture over the squash, cover with foil, and bake for 1 hour.
Remove the foil, sprinkle the remaining Gruyère on top, and bake uncovered for another 15 minutes, or until golden and bubbling.