Potage Parmentier — Leek and Potato Soup

Ingredients

• 400 g leeks, white and pale green parts only, finely sliced into rounds
• 500 g floury potatoes, Maris Piper, King Edward, or Bintje — peeled, cut into even 2 cm dice
• 50 g unsalted butter, for sweating
• 1.2 litres chicken stock or water
• 100 ml double cream, plus a little extra to garnish
• 30 g cold unsalted butter, cut into small pieces, for finishing off the heat
• 1 tsp fine salt, to taste; white pepper to taste
• To garnish
• 1 small bunch chives, cut into short batons
• 2 slices white bread, crusts removed, cut into small cubes for croutons

Method

1. 1
   Melt 50 g of butter in a heavy-based saucepan over medium-low heat. Add the sliced leeks and sweat them gently for 8–10 minutes, stirring occasionally, until completely soft and translucent. They must not take on any colour whatsoever — even the faintest browning will shift the soup's character from pure white elegance to something altogether more rustic. The leeks are ready when they yield without resistance and carry no trace of gold.
2. 2
   Add the diced potatoes to the pan and pour over the 1.2 litres of stock or water. Bring to a gentle boil, skimming any foam that rises, then reduce to a steady simmer. Cook for 20–25 minutes. The potatoes are done when they collapse completely under light pressure from a spoon.
3. 3
   Pass the contents of the pan through a moulin-légumes fitted with its fine disc — the preferred instrument, which renders the soup silky yet textured without overworking the potato starch. If a blender is used, work in short pulses only. Pass through a fine sieve if absolute smoothness is required. The purée should be uniform and pale ivory in colour.
4. 4
   Return the purée to a clean pan over gentle heat. Adjust the consistency with additional stock as needed — the soup should coat the back of a spoon but flow freely when the spoon is tilted. It is ready when it holds a clean line drawn across the spoon's back.
5. 5
   Remove the pan from the heat entirely. Stir in the 100 ml of double cream, then swirl in the 30 g of cold butter in small pieces until fully incorporated. Season precisely with salt and white pepper — potato soups absorb salt generously and will taste flat if under-seasoned. Taste once more; the balance should be clean, round, and faintly sweet from the leek.
6. 6
   Ladle into warmed bowls. Finish each portion with a thin drizzle of cream, a few chive batons, and, if desired, croutons fried in butter until golden and crisp — the sound of them crackling against the spoon signals they are correctly made.