2 tablespoons butter
1 medium onion, chopped
1/4 cup all-purpose flour
1/2 teaspoon salt
2 cups milk
2 cups chicken broth
1 head cauliflower, cut into 1-inch pieces
1 teaspoon Dijon mustard
2 cups shredded sharp Cheddar cheese
1. In 4-quart saucepan, 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.