Wisconsin Cauiflower soup

This is a Zupa's copycat recipe my whole family enjoys.

2 T butter
1 onion, chopped

1:4 c flour
1/2 t salt

2 c milk
1 can chicken broth
1 1/2 c water

1 head cauliflower cut into 1 inch pieces. Add and simmer until tender (15 min)
Blend in blender, then add
1 t  Dijon mustard
1 c shredded cheddar
1 c pepper jack