Fill a large pot with water and bring to a boil.  Add potatoes and boil for 10 minutes.  Drain potatoes and set aside. 

In the same pot over medium heat, melt the butter then saute the onions until translucent.  Add the flour stirring until incorporated with the butter and onions.  Add 1 cup of milk, you will have a nice thick rue.  Slowly add the rest of the milk, turn the heat to medium high stirring constantly.  Once it comes to a boil turn the heat down to low and simmer until liquid thickens, then add your potatoes. 

I usually simmer for about 10 minutes. Add garnish if desired. 

We usually eat this with saltine crackers.