Watercress and Potato Soup

From the garden

• 8 oz watercress, coarse stems trimmed, the rest roughly chopped · about 6 packed cups; the tender stems carry as much flavor as the leaves
• 2 medium leeks, white and pale green parts sliced, then washed well
• ¾ lb floury potatoes, peeled and diced small

From the pantry

• 3 tbsp butter
• 4 cups vegetable stock
• ⅓ cup heavy cream (optional) · stirred in off the heat; the soup is good without it
• 1 lemon, juiced · a squeeze at the end, to taste
• salt · to taste
• black pepper

1. Melt the butter in a soup pot over medium heat and sweat the leeks with a pinch of salt for 6–7 minutes, until soft and sweet with no color on them.
2. Add the potatoes and the stock, bring to a boil, then simmer 12–15 minutes until a knife slides through a cube of potato with no resistance.
3. Chop the watercress only now, holding back a few whole sprigs for the top, because bruised cress loses its snap within minutes.
4. Push the entire pile of cress into the pot, count to thirty, and blend at once with a stick blender (or in batches, lid vented, in a jug blender). Any longer in the heat and the soup turns olive drab instead of brilliant green.
5. Stir in the cream and a squeeze of lemon off the heat, season with salt and plenty of black pepper, and scatter the reserved raw sprigs over each bowl. Cress cut from a clean home tub is safe raw; anything gathered from a wild stream goes into the pot to cook, never on top.