Place potato and garlic in a saucepan, and cover with water. Bring to a boil, and cook 8 minutes or just until tender. Drain.
While the potatoes are cooking, place bread in food processor; pulse 10 times or until coarse crumbs form to measure 1/2 cup. Combine the breadcrumbs and butter, and set aside.
Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a medium, heavy saucepan; gradually add milk, stirring with a whisk until blended. Place over medium heat; cook until thick (about 8 minutes), stirring constantly. Remove from heat; add cheese, salt, and pepper, and stir until melted. Arrange potato and garlic in an 11 x 7-inch baking dish coated with cooking spray. Pour cheese sauce over potatoes. Sprinkle with breadcrumb mixture. Bake at 350° for 35 minutes or until bubbly. Let stand 10 minutes.