To make walnuts, add olive oil to a skillet and warm over medium heat. Add walnuts and stir, then cook for 5 minutes. Add butter, maple syrup, rosemary, and salt. Continue cooking for another 3-4 minutes, until you can smell the nuts giving off a toasty smell. Remove them from pan and set aside.
Preheat oven to 375 degrees F.
Place squash halves and garlic on a large baking sheet lined with parchment paper. Drizzle with olive oil. Flip squash halves cut side down.
Bake for 40 minutes, or until squash is soft and can be pierced through with a fork. Slide seeds out with a spoon and set aside. Scrape flesh out of squash, and squeeze garlic out of it's roasted skin. Place together in a large bowl. Add butter, half and half, salt, pepper, apple cider vinegar, rosemary, and maple walnuts. Mash with a fork and stir together, until smooth.
Top with chopped walnuts, a drizzle of maple syrup, and more chopped rosemary. Serve warm. Leftovers will stay good in refrigerator for up to three days.