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 degrees.
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 degrees for 20 minutes or until bubbly and golden.