Combine first 4 ingredients in a small saucepan; bring to a boil. Remove from heat. Cover; let stand 10 minutes. Pour milk mixture through a sieve over a large bowl, reserving liquid; discard solids.
Preheat oven to 350°.
Place potato in a medium saucepan. Cover with water; bring to a boil. Reduce heat. Simmer 15 minutes or until tender; drain. Place in a large bowl; set aside.
Melt margarine in a saucepan over medium heat. Add shallots; saute 3 minutes or until soft. Stir in flour; gradually add reserved milk. Stir with a whisk until blended. Bring to a boil. Cook 1 minute or until slightly thick; stir constantly. Remove from heat; stir in cheese, salt, and pepper. Pour milk mixture over potato; toss. Spoon potato into an 11 x 7-inch baking dish coated with cooking spray. Bake at 350° for 20 minutes. Broil 5 minutes or until lightly browned.