• 60 g/ 2 oz unsalted butter
• 450 g/ 1 lb potatoes, diced
• 1 leek, thinly sliced
• 225 g/ 8 oz onion, diced
• salt and pepper
• little milk (optional)
• 60–85 g/ 2–3 oz rocket leaves, finely chopped

Melt the butter in a heavy saucepan over a moderate heat and sweat the potatoes, leek and onion for 2–3 minutes, stirring constantly with a wooden spoon and taking care not to brown the vegetables.

Barely cover with about 1.1 litres/ 2 pints of cold water and bring to the boil. Lower to a bare simmer, season with salt and pepper and cook until the potatoes are just done.

Purée in a blender or food processor and return to the pan. You can hold at this point for several hours before reheating gently until hot. If the soup is too thick at this point, thin it with a lithe milk until you have the consistency of single cream.

Just before serving, stir in the chopped rocket leaves. Remove from the heat, taste, adjust for seasoning and serve at once.