In a skillet, melt butter over medium-low heat. Add the thinly sliced onions and sauté until they turn golden and release a sweet aroma, about 20 minutes.
Peel the butternut squash, remove seeds, and cube the flesh. In a pot, bring the broth to a boil and cook the squash until it’s tender, which should take about 5-7 minutes.
After the squash is cooked, reserve 1/2 cup of the broth and drain the rest. Transfer the squash to a blender, add the caramelized onions, milk, salt, and reserved broth.