Preheat the oven to 400°F (200°C) for roasting the squash.
Prepare the butternut squash by peeling, cubing, and tossing with olive oil, salt, and pepper.
Spread the seasoned squash evenly on a baking sheet and roast for about 25–30 minutes, stirring halfway through.
While the squash roasts, prepare the other salad ingredients: wash and dry the spinach, chop the walnuts, and slice the red onions.
In a large salad bowl, combine the spinach, roasted squash, feta cheese, walnuts, cranberries, and red onions.
Drizzle with balsamic vinaigrette and toss gently to combine.
Serve immediately or let sit for a few minutes for flavors to develop.