To prepare duxelles, place the mushrooms in a food processor, and process until finely chopped.
Melt 1 tablespoon butter in a large nonstick skillet over medium heat. Add onion, and cook 3 minutes, stirring frequently. Add mushrooms, thyme, 1/4 teaspoon salt, 1/4 teaspoon pepper, and garlic, and cook 5 minutes, stirring occasionally. Combine 1/2 cup milk and flour, stirring with a whisk. Add milk mixture to pan; bring to a boil. Cook 1 minute or until thick, stirring constantly.
To prepare the potatoes, place potato in a medium saucepan, and cover with water. Bring to a boil. Reduce heat, and simmer for 15 minutes or until tender. Drain and return potato to pan. Add 3/4 cup milk, 3 tablespoons butter, 1 teaspoon salt, 1/4 teaspoon pepper, and egg; mash with a potato masher.
Spread half of potato mixture evenly in a 2-quart microwave-safe baking dish coated with cooking spray, and top with mushroom mixture. Spread remaining potato mixture over mushroom mixture. Cover with plastic wrap, and vent. Microwave at HIGH 8 minutes or until thoroughly heated. Garnish with parsley, if desired.