Preheat the oven to 350F degrees. Cut the butternut squash in half and scoop out the seeds. Place 2 tablespoons butter, the onion, and broth in a casserole dish. Place the squash halves over the onion mixture and roast, uncovered, until the squash is soft, about 45 minutes to 1 hour.

Puree the squash mixture in a blender or food processor and pour into a saucepan. Add the evaporated milk and cinnamon and simmer for 5 minutes. Saute the red onion in 3 tablespoons butter in a skillet over medium heat for 5 to 6 minutes. Add the sugar and saute until caramelized, about 5-6 minutes. Garnish each serving of soup with caramelized onions.