Place water in soup pot over high heat. Add carrots, celery and green onions; bring to boil for 5 minutes; set aside, but do not drain.
Melt butter in a large stockpot over medium heat and add onion; saute 1 minute, then add flour, blending well.
In a large saucepan, bring milk and broth to boil. Whisk broth mixture into flour mixture with a wire whisk.
Stir in cheese, salt, pepper and cayenne. Stir in mustard and the cooked vegetables, including the water in which they were cooked. Bring to a boil and serve immediately.
Recipe Tips
What to serve with this soup:
I would really recommend serving this soup with a glass of white wine. A refreshing riesling would be perfect.
Fresh hot bread like my light and airy popovers would also be perfect.