Bring broth to a boil. Add carrots and cook for 5 minutes. Then add cauliflower and cook for additional 5-10 minutes or until fork tender. Drain, reserving 1/2 cup broth. Transfer carrots and cauliflower to the blender. Add the onions, garlic, milk, salt, mustard, 1/2 cup cheddar, and reserved broth and puree until smooth and creamy. Add additional salt and pepper to taste.