Wisconsin Cauliflower Soup (like Zupa's)

Made this last night and took some over to our Lady-in-waiting Meghan. It's good, I'm having some for lunch.

2 Tbsps butter (1/4 stick)
1 medium onion, chopped
1/4 cup flour
1/2 tsp salt
2 cups milk (I used half & half)
1 can chicken broth (I used low sodium)
1 head (2 1/2 pounds) cauliflower, cut into fairly small pieces
1 tsp Dijon mustard
1 package (8 ounces) shredded sharp Cheddar cheese (about 2 cups)

In saucepan, melt butter over med heat. Add onion and cook until golden, about 10 min, stirring occasionally. Stir in flour and salt; cook 2 minutes, stirring frequently.

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.

In blender (remove center part of blender cover to allow steam to escape), blend cauliflower mixture at low speed in small batches until very smooth.

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 w/ oyster crackers on top.