Place sun-dried tomatoes in a small bowl.

Cover with boiling water; let stand for 5 minutes.

Drain the tomatoes and set aside.

In a large skillet over medium heat, cook sliced onion in butter until tender.

Add minced fresh oregano, salt, pepper, and the reserved tomatoes.

Cook for 2 minutes longer.

Sprinkle with all-purpose flour; stir until blended.

Gradually add fat-free milk.

Bring to a boil; cook and stir for 2 minutes or until thickened.

Remove from the heat; stir in grated Parmesan cheese.

Place thinly sliced potatoes in a greased 13-in. x 9-in. baking dish.

Top with the sauce.

Cover and bake at 350°F (175°C) for 1 hour.

Uncover; bake for 10-15 minutes longer or until potatoes are tender.