Heat the oven to 425°F. Toss squash with 1 tablespoon oil, 1/2 teaspoon salt and pepper on a large rimmed sheet, spreading into one layer. Roast for 30 to 35 minutes, turning once, until tender and browned at the edges.
Meanwhile, heat remaining oil in a soup pot over medium heat. Cook onion for 7 to 8 minutes until soft, then stir in garlic for 30 seconds.
Add roasted squash, broth and remaining salt. Simmer gently for 10 minutes, then remove from the heat and blend with an immersion blender until completely smooth.
Melt butter in a small light-colored saucepan over medium-low heat for 3 to 5 minutes until its milk solids turn golden brown. Stir in chopped sage for 15 seconds and immediately pour the butter into the soup.
Stir in cream and vinegar. Warm over low heat for 2 to 3 minutes without boiling, stirring until evenly combined, then divide among 6 bowls.