Put chopped potatoes and onion, water or chicken stock in a deep pot. Season with salt and pepper.
Bring it to boil. Simmer over medium low heat until potatoes are very soft.
Meanwhile, in a medium saucepan melt butter over medium heat, add flour and cook for 1 minute while whisking constantly. While whisking, slowly add milk and cook, stirring constantly until mixture begins lightly bubble and thickens.
Pour this into the pot and blend it using a blender or in a food processor.