In a large saucepan add potatoes and 3 cups water. Cook, covered, over medium-high heat until tender, about 10 minutes; drain and set aside.

In a large non stick frying pan melt margarine over medium-high heat. Add carrots, onion, and garlic; cook, stirring frequently, until barely tender, about 5 minutes. Add mushrooms; cook and stir until mushrooms are tender, about 5 minutes.

Stir in sherry; cook for 1 minute. Remove 1-1/2 cup of the mushroom mixture; set aside.

In a food processor place potatoes and remaining mushroom mixture from frying pan. Process until smooth. Pour into saucepan. Add milk, chicken stock, thyme, pepper and reserved 1-1/2 cup mushroom mixture. Simmer over medium-heat until heated through, about 10 minutes.