Preheat oven to 230C. Boil fennel bulb, trimmed and split lengthwise, until tender, about 15 minutes. Drain fennel, cut side down, on paper towels, 5 minutes. Place fennel, cut side up, in a buttered baking dish and brush with 1 tablespoon softened butter. Season with coarse salt and ground pepper and top with 50g grated Parmesan and 4 sprigs thyme. Bake until cheese is golden brown, about 20 minutes.