1Heat oven to 350°F. Spray 1 1/2-quart casserole with cooking spray.
2In 12-inch nonstick skillet, melt butter over medium heat. Cook onion in butter 3 to 4 minutes, stirring frequently, until crisp-tender. Stir in flour and Cajun seasoning until combined. Slowly pour milk into onion mixture, stirring constantly until mixture is well combined and slightly thickened.
3Reduce heat to medium-low. Stir in Cheddar cheese and 1/2 cup of the Parmesan cheese until melted. Fold in potatoes until evenly coated with cheese sauce. Spoon mixture into casserole.
4Bake uncovered 30 minutes. Sprinkle with remaining 1/4 cup Parmesan cheese. Bake 14 to 17 minutes longer or until potatoes are tender and cheese is melted.