This is adapted from the Moosewood County cookbook.
Melt the Butter in a large soup pot. Add onion and carrot, and sautè 5 minutes. Add the sweet potato, cauliflower, salt and water. Bring to a boil, reduce heat, and simmer 20 minutes.
Remove pot from heat. Add the cream, dill, and mustard. Blend the soup with an immersion blender. (Alternatively, you can blend it in 3 batches in a blender and then return to the pot.)
Gently stir in the cheese.