An invention of Louis Diat in the early 20th century whilst working at the Ritz-Carlton in Hotel in New York, he would describe it as coming from his mother's recipe because she would add cold milk to a leek and potato soup. Their home, in Bourbonnais, was approximately twenty miles from the Vichy spa.

The ingredients are simple; white potatoes, leeks, and onions. Peel the potatoes beforehand (save the skins for another dish), use white onions and, for goodness sake, only the white of the leek. I have seen some recipes use the green part of a leek. Save that for a colcannon).

Cook the soup in light (single) cream and milk for 30 minutes. Purée soup in a blender or food processor. Allow to cool to room temperature, then add in double cream (or ricotta cheese!), and chopped chives are added before serving.

In summer this can be served chilled.

Ingredients
1 tablespoon butter
3 leeks, whites only, sliced into rings
1 onion, sliced
5 medium potatoes, peeled and thinly sliced
salt and pepper to taste
1/2 teaspoon dried marjoram
1/4 teaspoon dried thyme
5 cups vegetable stock
milk, single cream, double cream or ricotta cheese.