Arrange a rack in center of oven; preheat to 400º. On a large baking sheet, toss squash and potatoes with 2 Tbsp. oil; generously season with salt and pepper.
Roast squash and potatoes, stirring halfway through, until tender, 30 to 35 minutes.
Meanwhile, in a large pot over medium heat, heat butter and remaining 1 Tbsp. oil until butter melts. Cook onion, celery, and carrot, stirring, until softened, 7 to 10 minutes. Stir in thyme; generously season with salt and pepper.
Add roasted squash and potatoes, then pour in broth. Bring to a simmer and cook, stirring occasionally, until slightly reduced and flavors have melded, about 10 minutes.
Using an immersion blender, blend soup, adding more broth if needed, until creamy. Divide among bowls. (Alternatively, carefully transfer batches of the hot soup to a standard blender and divide among bowls.)
Top with more thyme. Drizzle with cream (if using).
