Spread the squash over a large baking sheet and drizzle with olive oil.
Sprinkle squash with salt and toss to combine. Spread out the squash again on the baking sheet.
Roast the squash for 30-40 minutes, or until tender and the edges are starting to turn golden brown.
Meanwhile, in a sauté pan, heat 1 Tbsp. of olive oil over medium heat. Add onion, lower heat to med-low, and stir until caramelized. Season with salt (optional).
When the squash and onions are finished cooking, let them cool and add to blender or food processor + 3 cups of vegetable broth + 2-3 tsp. of balsamic vinegar. Puree until smooth!
Season as needed with additional salt, pepper, or balsamic vinegar.