Preheat oven to 400 degrees F.
On a parchment lined baking sheet, combine the butternut squash, sliced onions, olive oil, salt and pepper and toss.
Evenly arrange in a single layer on a baking sheet.
Roast about 35-45 min, until lightly browned and tender.
Remove from oven and add the baby spinach, return to the oven for about 5 minutes or until spinach slightly wilts.
Serve and enjoy.