Peel potatoes, chop into small pieces, and place in a small saucepan with the water. Cover, bring to a boil, and simmer until potatoes are tender.
Meanwhile, melt butter in a deep skillet or Dutch oven. Add onion and celery, and sauté over medium heat for about 5 minutes. Stir in mushrooms and salt, cover, and cook about 10 minutes, stirring occasionally. Somewhere in there, add garlic and thyme. (Adding garlic later keeps its flavor stronger.) Remove from heat; stir in sherry, soy sauce, and pepper.
Use a blender or food processor to purée both the potatoes in their cooking water, and the mushroom sauté in its own liquid. (When you run out of the liquid, use the milk.) Combine the purées in a kettle or Dutch oven.
Stir, and heat gently; taste to adjust seasonings.
Serve very hot, with optional, but highly recommended, toppings.