STEP 1: Make the soup base.

In a large saucepan, cook the bacon over moderate heat,stirring until it is browned slightly; add cabbage, onion, leek and celery and cook, stirring, for 5 minutes. Add the stock and bring to a boil, skimming the froth. Add the potatoes and simmer the mixture for 15 minutes. In a bowl, whisk together the milk and flour until the mixture is smooth. Add the mixture to the saucepan in a stream, stirring, and bring liquid to a boil. Simmer the mixture for 35 minutes;season with salt and transfer to a large bowl. Chill the soup base, covered. Makes about 10 cups.The soup base may be kept chilled in an airtight container for up to 3 days.

In a blender at high speed or in a food processor fitted with a steel blade, blend 4 cups of the soup base,the fiddleheads, the sour cream and lemon juice until it is smooth. Season with salt and pepper, ladle into chilled bowls and garnish with a dollop of sour cream and reserved fiddleheat tips.

Makes 6 cups,serving 4 to 6.

NOTE: the soup base is good on its own for a soup dish.