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 42
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.