Heat oven to 425°F. On a large rimmed baking sheet, toss squash with 1 tablespoon oil, 1/2 teaspoon salt and pepper. Spread in one layer. Roast 30–35 minutes, turning once, until tender and browned.
Meanwhile, heat remaining oil in a soup pot over medium. Soften onion, 7–8 minutes. Stir in garlic for 30 seconds.
Add squash, broth and remaining salt. Simmer gently 10 minutes. Remove from heat; blend smooth with an immersion blender.
Brown butter in a small, light-colored saucepan over medium-low, 3–5 minutes. Stir in sage for 15 seconds; immediately pour into soup.
Stir in cream and vinegar. Heat on low, stirring, 2–3 minutes. Don't boil.