Melt butter in a heavy bottom saucepan and add carrots. Cook over medium heat for 10 minutes, stirring occasionally. Add orange juice, vegetable bouillon and onion.
Bring to a boil and season with thyme, salt and pepper. Reduce heat, cover and cook for 20 minutes until carrots are tender. Remove from heat and cool.
Blend soup in batches. Heat and serve in individual soup bowls or plates. Place 1 tablespoon of sour cream in the center and sprinkle with nutmeg.
Garnish with a fresh thyme sprig if desired.