Jacques Pepin is a French master chef, author of many books, and host of a PBS cooking show. He’s lived in Connecticut for many years, long enough that he’s certainly an American (and has quite possibly reached New Englander status). An old-fashioned food mill is a great tool for pureeing this soup, but we’ve found that an immersion blender is even easier.
Pick over watercress, discarding any yellowed leaves. Chop coarsely. You should have 3-4 cups greens after chopping.
In a large saucepan over medium heat, melt 1 tablespoon butter. Add onion, leek, and watercress. Cook, stirring often, until all ingredients become soft, about 5 minutes.
Add potatoes, stock, salt, pepper, and thyme. Increase heat and bring to a rolling boil. Reduce heat, cover, and let simmer 1 hour.
Using a food mill or immersion blender, purée the soup. Return purée to pan.
Add milk and cream. Bring to a boil, stirring slowly. Adjust seasoning with salt and pepper if needed.
Just before serving, stir in remaining 1 tablespoon butter, bit by bit, stirring well after each addition until butter is well incorporated into soup. Don’t let it boil. If soup is too thick for your taste, adjust with milk or water. Serve hot or cold.