Preheat oven to 400°F // 200°C. Line a baking sheet with parchment paper, and set aside.
Slice the Hubbard squash in half, lengthwise, and scoop out seeds. (Reserve the seeds for roasting)
Drizzle olive oil over both halves of the squash, and season with salt and pepper. Place squash, cut side down, on parchment-lined baking sheet.
Roast squash for 45 - 60 minutes. (Test for doneness by flipping over and piercing flesh with a paring knife - it should have very little resistance.)
Remove squash from oven and let cool slightly. When it is cool enough to handle, scoop flesh from the skin into a large mixing bowl. Discard skin.
Add salted butter, maple syrup and nutmeg to warm squash and mash until desired consistency with a potato masher. You can also use a handheld electric mixer if it is easier for you.
Taste and adjust seasoning, if necessary. Serve.