Barely cover potatoes and onions with water. Add salt and simmer until potatoes are done - approximately 15-20 minutes. Add the 4 cups of milk and slowly bring to a boil. Make thickening. Whisk 1/2 cup flour into hot, melted butter to combine. Cook on stove for a minute, then mix in enough warm water to make into a loose paste. Add slowly to soup. May need to use whisk again if it is too lumpy. Add pepper to taste. Serve with grated cheese and crumbled bacon, if desired.