Pre-heat the oven to 400 degrees. Line a large sheet pan with parchment paper for easy cleanup. (You can also use aluminum foil, but make sure to spray it with cooking spray first. A silicone baking sheet is great as well.)
Add the squash, sausage, and onion to the prepared sheet pan. Drizzle with olive oil and season with a good pinch of salt, black pepper, and cinnamon (if using), and toss to coat.
Roast for 25 to 30 minutes, stirring once halfway through the cooking time, until the squash and onions are tender and the sausage is browned. (Keep an eye on them during the last 10 minutes to make sure things aren't burning.)