Prepare white sauce over medium/Low setting. Melt butter in a saucepan, add flour and salt until creamed together. Add milk a little at a time while constantly stirring with a whisk. Stir till mixture becomes thick and smooth.

Place butter in a saucepan on med. heat. Add onions and cook till transparent. Place spinach in pan and add water. Lower heat and place lid on pan. Stir occasionally until spinach is almost completely cooked. Add salt, white sauce, and sour cream. Stir well and simmer until completely blended.