Combine the cauliflower, carrot, celery, and chicken stock in a large pot and bring to a simmer. Reduce heat; cover and simmer for 12-15 minutes or until vegetables are tender.
Blend vegetables with immersion or food blender.
Melt butter.In another large saucepan. Stir in the flour and stir until smooth. Gradually add milk. Bring to a boil over medium heat; cook and stir for 2 minutes or until thickened. Reduce heat.
Stir in the cheese and sour cream until melted.
Add to the cauliflower mixture and season to taste.