Strain the oysters and their liquor into separate containers and keep both refrigerated until ready to use.
In a large, heavy-bottomed saucepan, melt the butter over medium heat until foaming. Add the leek and celery and saute until softened and translucent, about 8 minutes–do not brown. Stir in the milk, cream, and oyster liquor into the saucepan with the vegetables. Do not allow the liquid to boil. When the soup base is simmering, add the oysters and poach them until they begin to shrink and curl on the edges, about 2 minutes. Season with salt and pepper.
Divide the stew into four bowls and add sherry (if desired) to each bowl. Garnish with chives and serve crackers on the side.