Clean asparagus and remove tips. Blanch and shock the tips. Set aside. Sauté onions and leeks in butter in soup pot until translucent. Dust with flour to make roux, stir.Add stock, potatoes, asparagus stems, bay leaf, black pepper and salt. Bring to a boil, reduce heat. Simmer for 1 hour.Remove bay leaf, puree and strain. Garnish with blanched asparagus tips and serve.