Melt butter in a saucepan over low heat and stir in flour till you have no lumps. Cook 2 minutes. Add salt & pepper then mix in sour cream. Slowly add in water till combined. Stir on medium heat till glossy and smooth. Away from heat add dill and pour slightly cooled sauce on cauliflower.
Bake in oven about 40-45 minutes till cauliflower is tender to knife insertion.
** Optional, add shredded cheese to top in last 3 minutes of cook time.