Place a rack in the upper third of the oven; preheat the oven to 425°. Using a peeler or a pairing knife, remove hard skin and a thin layer of white flesh below. Halve the squash lengthwise and scoop out the seeds.
Using a large sharp knife, score rounded sides of squash crosswise, going deep but not-quite-all-the-way through. Rub the squash with oil; season with salt and pepper. Stick the sage leaves in between the cuts. Place the squash into a baking dish, flat side down, and roast in the oven for 15 minutes.
Meanwhile, melt the butter in a small saucepan. Continue cooking the butter over medium heat, stirring frequently, until it gets golden and starts smelling nutty. It will take about 5 minutes. Remove from the heat immediately, or it might burn, and stir in the maple syrup.
After 15 minutes of roasting, start brushing the squash with the maple-butter sauce. Do it every 15 minutes until the squash is very tender and caramelized, about 45 to 60 minutes. Then, remove the squash from the oven.
Serve the butternut squash warm, topped with pomegranate molasses and pomegranate seeds.