In a large pot over high heat, combine the potatoes, onions and water to cover and boil for 30 to 45 minutes, or until tender. Drain the mixture, reserving 3 cups of the water. Transfer this in small batches to a blender and puree until smooth.
In the same pot over medium heat, combine the flour and the butter, stirring together well, to form a roux. Slowly add the milk, stirring constantly, until well blended. Reduce heat to low and add the pureed potato mixture. Let simmer, stirring occasionally for 5 to 10 minutes. Add the parsley and season with salt and pepper to taste.
