1. Preheat oven to 350°.
2. Bring 1 quart water to a boil; stir in rice. Cook 5 minutes; drain.
3. Heat a large skillet over medium-high heat. Add oil to pan; swirl to coat. Add onion; cook 6 minutes or until tender, stirring occasionally. Sprinkle flour and salt over onion mixture, stirring until blended. Cook, stirring constantly, 1 minute.
4. Gradually add milk, stirring with a whisk until blended. Cook 3 minutes or until thick, stirring frequently. Add Gruyère cheese and rice, stirring until cheese melts. Pour onion mixture into an 11 x 7-inch glass or ceramic baking dish coated with cooking spray. Sprinkle with Parmesan cheese.
5. Bake, uncovered, at 350° for 35 minutes or until bubbly and brown. Sprinkle with parsley.