Peel the potatoes and cut into thin slices (it's ok if you don't soak them), then thinly slice the onion.
Stir fry the onion and potato in the butter and vegetable oil, taking care not to let anything burn.
Add 2 cups of water along with the soup stock cube, 1/2 teaspoon salt and some pepper (white if you have it.) When the mixture has boiled, turn the heat to low and simmer for 15 minutes. Skim away any scum.
Add the milk and bring to a boil again, then stop the heat.
Blend with a hand mixer until smooth. When completely cooled, move to the fridge and chill well.
Serve in chilled cups and sprinkle some chives over to finish.
Story Behind this Recipe
I often make this when it's hot and I don't have an appetite. I recommend eating this vichyssoise well chilled.
For Step 5, use the hand mixer only when the mixture has cooled. If it isn't you could get hit by some hot soup (people who are used to that wll be OK though.) Since this soup is chilled I added a lot of salt. You can reduce the amount if you like.