01 - Melt butter in a large pot over medium heat. Add onion, carrot, and celery. Sauté for 5 minutes until softened and fragrant.
02 - Stir in minced garlic and cook for 1 minute until aromatic, being careful not to burn it.
03 - Add diced potatoes, thyme, nutmeg, vegetable stock, salt, and pepper. Bring to a boil, then reduce heat to low. Cover and simmer for 20–25 minutes until potatoes are completely tender.
04 - Use an immersion blender to puree the soup until completely smooth. For a chunkier texture, blend only half the mixture.
05 - Reduce heat to low. Stir in milk, heavy cream, grated cheddar cheese, and sour cream. Cook for 5 minutes, stirring constantly until cheese is fully melted and soup is creamy. Adjust salt and pepper to taste.
06 - Ladle hot soup into serving bowls. Garnish generously with extra cheddar cheese and chopped fresh chives.