Preheat the oven to 425°F Place the cauliflower, stem side down, in a large saucepan with 1/2-inch of water and cover. Bring to a simmer at medium-high heat for 4-5 minutes. Remove from heat and allow cauliflower to steam 1-2 minutes more, until tender.
Meanwhile, finely chop 1 tablespoons sage and set aside. Put remaining sage sprigs, onion and garlic in a roasting pan and drizzle with olive oil. Place the cauliflower on top. Combine the butter, nutmeg, salt and pepper and spread over the cauliflower. Roast in the top third of the oven for 25-30 minutes, until the cauliflower is golden.
Transfer the cauliflower to a serving platter. Squeeze the roasted garlic into the roasting pan and discard the skins. Add the reserved chopped sage and mix with the roasted garlic and onions. Spoon the mixture around the cauliflower and garnish with cheese and chives. To serve, cut the cauliflower into wedges, and top with the spoonfuls of the onion and garlic mixture.