Heat oil in large soup pan over medium-high heat. Add onion and a sprinkle of salt and sauté until onion is fragrant and translucent, 3-5 minutes.
Add minced garlic, chopped sweet potato, and dried mustard (if using) and sauté an additional 2-3 minutes, stirring frequently.
Add chicken stock and chopped cauliflower and bring to a boil.
Reduce heat, cover, and simmer for 15-20 minutes, until vegetables are tender.
Remove from heat, add cream cheese, and purée with an immersion blender or in batches in a regular blender. (If you want to keep some chunks, reserve 1 cup of the cooked cauliflower before puréeing the rest of the soup).
Return soup to heat, add Cheddar cheese, and stir until cheese is melted and soup is heated through, 2-3 minutes.
Season with salt and pepper to taste, garnish as desired, and serve.
Recipe by Hidden Ponies at https://hiddenponies.com/2012/05/cauliflower-sweet-potato-soup/