Melt butter in a large pot over medium-high heat. Add onion and garlic and sauté for 5 minutes until tender. Add carrots and sweet potatoes and sauté for 10 minutes more until vegetables just begin to soften. Season vegetables with salt and pepper.
Add water to the pot until it just covers the vegetables (about 4-5 cups) and bring to a boil. Cover the pot. Reduce heat to medium and simmer for about 30 minutes, until potatoes are very soft.
Using a hand blender, puree the soup in the pot until smooth. Stir in thyme, brown sugar, nutmeg, cream if desired, and salt and pepper to taste, and heat through. You can later discard thyme sprigs if you want.
Reheat soup before serving. Ladle into bowls and serve hot.