Peel and seed the pumpkin, then cut it into approximately 1-inch cubes. Peel and chop the carrot and red onion. Finely chop the garlic and ginger.
Heat the butter and olive oil in a Dutch oven or heavy-bottomed pot over medium heat.
Add the garlic, ginger, and sage. Cook for about 1 minute, stirring constantly, until fragrant.
Add the red onion and carrot. Cook for approximately 5 minutes, stirring occasionally.
Add the cubed pumpkin and cook for another 5–7 minutes, stirring occasionally.
Pour in the white wine and scrape up any browned bits from the bottom of the pot. Simmer uncovered for 3–4 minutes.
Add 2 cups of vegetable broth. Bring to a boil, reduce the heat, cover, and simmer for 15–20 minutes, until the pumpkin and carrot are completely tender.
Remove the pot from the heat. Carefully blend the soup with an immersion blender until smooth.
Add more broth as needed to reach your preferred consistency. Season with salt and black pepper.
Divide the soup among four bowls. Top with crumbled blue cheese and, if desired, crushed red pepper flakes. Serve immediately.