01 - Set oven to 400°F. Line a baking sheet for roasting.
02 - Toss butternut squash and carrot with 1 tablespoon melted butter, salt, and pepper. Spread evenly on baking sheet and roast for 25 to 30 minutes until tender and caramelized.
03 - In a large pot over medium heat, melt remaining 3 tablespoons butter. Add sage leaves and cook, stirring, until butter becomes golden and aromatic, about 3 to 4 minutes. Remove sage leaves using a slotted spoon; reserve for garnish.
04 - Add diced onion and minced garlic to the brown butter. Sauté until onion turns translucent, about 4 minutes.
05 - Add roasted squash and carrot to the pot. Stir in nutmeg, then pour in vegetable broth. Bring mixture to a boil, reduce heat, and simmer for 15 minutes.
06 - Remove pot from heat. Use an immersion blender or countertop blender in batches to blend until perfectly smooth.
07 - Stir in heavy cream, if using. Taste and adjust salt and pepper as needed.
08 - Ladle hot soup into bowls. Drizzle with additional browned butter and top with crispy sage leaves.