Start by cutting the acorn squash in half. Cut the squash along a depression line and carefully wiggle your knife down until it is cut through half of the squash.
Turn the squash around so the opposite end is facing you, and repeat the same process on the other half, wiggling the knife all the way to the stem end.
Use a large spoon to scoop out the seeds and discard.
Rub the squash halves with the oil and sprinkle with salt.
Line a 9x13" rimmed baking sheet or roasting pan with parchment paper and place the squash cut side down so that the flesh is flat against the pan. This helps trap steam and cook the squash more evenly.
Bake at 400F (204 C) for 45 minutes until soft.
While the squash is roasting, make the apple cider glaze. In a small saucepan, heat the apple cider, butter, mace, and nutmeg.
Simmer the apple cider mixture over medium heat until reduced by half and thickened. You should be able to see the bottom of the pot when scraping the glaze.
Remove the squash from the oven and set aside to cool for about 10 minutes. Reduce the oven temperature to 350F (176 C).
When the squash pieces are cool enough to handle, flip them over so the flesh is exposed.
Brush a generous amount of the glaze all over the squash halves using a pastry brush.
Return the squash to the oven and bake for 10-12 minutes until the edges are golden brown and starting to caramelize.
Serve the squash with the remaining glaze on the side, along with any optional toppings you desire.