Preheat oven to 400ºF (200ºC).
Place squash halves cut sides up on a baking sheet and brush the entire open surface of each half with olive oil. Drizzle half a tablespoon of maple syrup into the cavity of each piece then season with kosher salt and pepper.
Roast for 35-45 minutes until tender when pierced with a paring knife or fork. If the squash is smaller, start checking for doneness at the 35-minute mark.
Transfer roasted squash to a serving plate. Drizzle with a little more maple syrup, sprinkle with fresh rosemary and season with flaky salt. Serve hot.