Bring the chicken stock to a boil in a small saucepan. In a heavy -bottomed pot, melt 4 tablespoons of the butter over medium-high heat. Add the leeks and saute, stirring for 2 minutes. Turn heat down to medium and sprinkle the flour over the leeks. Saute, stirring for 2 minutes. Remove from heat and add the boiling chicken stock all at once. Beat vigorously with a whisk to incorporate the flour into the stock. Return to boil and add the celery. Reduce to a bare simmer, and cook for 30 minutes.
When ready to serve, divide the raw oysters and the cooked asparagus tips among 6 wide, shallow soup bowls. If you find the soup too thick, add some oyster liquor to thin it. Strain the soup, bring it to a boil, add the Pernod, and turn off heat. Swirl in the remaining 2 tablespoons of butter with a whisk. Season to taste and pour immediately over the oysters and asparagus in the soup bowls, keeping the oysters just above the surface of the veloute. Garnish with fresh tarragon leaves, if available.
As an appetizer: The best way to use this recipe, we feel. A great start to a late-winter/early-spring dinner that leads, perhaps, into a roast leg of lamb. A wonderful way to serve it is to place the bowls before your guests, with the oysters and asparagus already set in them. Then, bring a tureen to the table and ladle the veloute over all.
As a main course: If you simply increase the amount of oysters and asparagus-say 8 per person rather than 4-you'll have a light but filling main-course soup. Serve warmed French bread with it, and a light frilly salad tossed with fresh goat cheese.