Preheat oven to 400.
Coat a rimmed baking sheet with nonstick cooking spray.
Combine squash and onion in a mixing bowl.
Sprinkle rosemary over vegetables.
Drizzle mixture with olive oil and maple syrup; toss to coat fully.
Spread coated mixture on prepared baking sheet.
Season with salt and pepper, if desired.
Roast for 10 minutes.
Flip squash and onion with a spatula; roast for an additional 10-15 or until squash is tender.
At this point you have two options:
Transfer squash mixture to a serving bowl, top with feta, and serve.
Allow squash mixture to cool slightly, transfer to a microwave-safe, sealable container, and refrigerate. Just before you’re ready to serve, reheat squash in microwave for 2-3 minutes with lid slightly open, tossing once or twice for even heating. Top with feta, and serve.