Combine water and sun-dried tomatoes in a bowl; cover and let stand 30 minutes or until soft. Drain and coarsely chop; set aside.
Preheat oven to 350°.
Place the potato in a large saucepan, and cover with water; bring to a boil. Reduce heat; simmer for 15 minutes or until tender. Drain well. Arrange potato in a 13 x 9-inch baking dish coated with cooking spray.
Melt butter in pan over medium heat. Add onion; cook 3 minutes or until tender. Add sun-dried tomatoes, oregano, salt, and pepper, and cook 2 minutes. Lightly spoon flour into a dry measuring cup; level with a knife. Sprinkle tomato mixture with flour; cook 1 minute, stirring constantly. Gradually add milk, stirring with a whisk until blended. Remove from heat; add cheese, stirring until cheese melts. Pour the sauce over the potatoes, tossing gently to coat. Bake at 350° for 20 minutes or until bubbly and golden.