Place potato in a saucepan; cover with water. Bring to a boil; cook 15 minutes or until tender. Drain and cool. Arrange 2 cups potato in an 11 x 7-inch baking dish coated with cooking spray.
Preheat oven to 425°.
Combine the milk and flour in a small bowl; stir with a whisk. Melt butter in a medium saucepan over medium heat. Add the onions, curry powder, and garlic. Cook 1 minute. Stir in milk mixture, and cook until thick (about 1 minute), stirring frequently. Remove from heat. Stir in the yogurt, tomato sauce, and salt. Spoon half of sauce over potato. Repeat the procedure with the remaining potato and sauce. Sprinkle with cheese. Bake at 425° for 30 minutes or until golden brown. Let stand 10 minutes.