Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone mat.
- Toss the diced butternut squash and sliced onions with 1 tablespoon of olive oil, salt, pepper, and cinnamon (if using). Spread them out evenly on the prepared baking sheet.
- Roast for 25-30 minutes, stirring halfway through, until the squash is tender and slightly caramelized and the onions are golden.
- While the squash and onions are roasting, heat the remaining tablespoon of olive oil in a large skillet over medium heat. Add the spinach and sauté for 2-3 minutes, until just wilted. Remove from heat.
- Once the squash and onions are done, transfer them to the skillet with the spinach. Add the dried cranberries and toss everything together.
- Drizzle with balsamic vinegar (if using) and sprinkle with fresh herbs (parsley or thyme), then toss again.
- Serve immediately as a side dish or light main course.