Wisconsin Cauliflower-Cheddar Soup

They serve this at a Zuppas - a favorite restaurant of mine. I've made this a few times and it is pretty close to what they serve.

2 tablespoons margarine or butter (1/4 stick)
1 medium onion, chopped
1/4 cup all-purpose flour
1/2 teaspoon salt
2 cups milk
1 can (13 3/4 to 14 1/2 ounces) chicken broth
1 head (2 1/2 pounds) cauliflower, cut into 1-inch pieces
1 teaspoon Dijon mustard
1 package (8 ounces) shredded sharp Cheddar cheese (2 cups)

1. In 4-quart saucepan, melt margarine or butter over medium heat. Add onion and cook until golden, about 10 minutes, stirring occasionally. Stir in flour and salt; cook 2 minutes, stirring frequently.
2. Gradually stir in milk, chicken broth, and 1 1/2 cups water; add cauliflower and heat to boiling over high heat. Reduce heat to low; cover and simmer until cauliflower is tender, about 10 minutes.
3. In blender (with center part of blender cover removed to allow steam to escape), blend cauliflower mixture at low speed in small batches until very smooth.
4. Return cauliflower mixture to saucepan; heat over medium heat until hot, stirring occasionally. Remove saucepan from heat; stir in mustard and 1 1/2 cups cheese until melted and smooth. Garnish soup with remaining cheese to serve.