Preheat oven to 400°F. Place winter squash, red onion, and walnut halves in a large mixing bowl.
In a separate bowl, whisk together the avocado oil, maple syrup, salt, and crushed red pepper flakes. Pour mixture over squash and toss until well coated.
Spread squash out on two parchment-lined baking pans in a single layer. Bake for 40 minutes or until squash is tender.
Place squash in a serving vessel and sprinkle on crumbled feta.