Set a colander over a small bowl. Add the oysters into the colander, reserving the liquor. Set a fine mesh sieve over another small bowl and pour the oyster liquor over the sieve. This will remove any sand or grit from the liquor. Discard solids. In a medium saucepan, melt the butter over medium heat. Add the shallots and celery, stirring for 2-3 minutes until vegetables are slightly softened and translucent. Turn the heat down to medium low and add the strained oyster liquor, milk, cream and thyme. Heat slowly until broth reaches a simmer (just a few bubbles rimming the pan) Do not boil, to avoid curdling the milk. Taste the broth and adjust seasoning, adding salt and pepper to taste. Slip the oysters into the broth and let them simmer for 2-3 minutes, just until the edges start to curl. You want the oysters to be tender, not tough - overcooking will make them rubbery. Ladle the stew into bowls and garnish with celery leaves and/or other optional garnishes.