In large pot, heat oil or butter over medium heat. Add onion and salt. Cook until onions are translucent, stirring occasionally.
Gradually stir in milk and chicken broth. Bring to boil.
Add cauliflower florets and sweet potato. Reduce heat and simmer until cauliflower is tender approximately 10 minutes.
Place mixture in blender with center piece out (so the steam can escape) and blend cauliflower mixture at low speed until very smooth.
Return cauliflower mixture to sauce pan. Over medium heat, add mustard and cheese stirring constantly. Once cheese is melted, remove from heat.
Garnish soup with additional cheese, if needed.
Additional tips: Leave some of the cauliflower florets in the soup to make it chunkier. You can also add broccoli, petite peas, carrots, or any of your favorite vegetables for added flavor.