While chestnuts are soaking, place potato in a medium saucepan, and cover with water; bring to a boil. Reduce heat, and simmer 15 minutes or until tender; drain.
Drain chestnuts.
Cut a slit in the shell on the rounded side of the chestnut. (Make sure slit goes the whole way though the shell. If not, they will explode.) Arrange chestnuts in a single layer on a microwave-safe dish. (Microwave a maximum of 12 chestnuts at one time, in order to cook evenly). Microwave at HIGH 2 minutes. Cool 5 minutes. Peel and chop to equal 2 cups.
Drain oysters in a colander over a bowl, reserving 1/2 cup oyster liquid; set oysters aside.
Cook bacon in a Dutch oven over medium heat until crisp.
Remove bacon from pan; crumble.
Add chestnuts, chopped onion, celery, and green onions to bacon drippings in pan; saut 5 minutes. Lightly spoon flour into a dry measuring cup; level with a knife. Stir in flour; cook 1 minute.
Add potato, reserved oyster liquid, milk, bacon, salt, thyme, and pepper. Bring to a boil; reduce heat, and simmer until thick (about 8 minutes). Stir in sherry and oysters; cook 3 minutes or until edges of oysters curl.
