Preheat oven to 350°F. Mix cornstarch, pepper, garlic powder, parsley and salt in small bowl. Set aside.
Melt butter in large skillet on medium-high heat. Add onion; cook and stir 3 to 4 minutes or until golden brown. Stir in cornstarch mixture. Add stock and milk; cook until sauce is slightly thickened and bubbly, stirring constantly. Remove from heat. Stir in sour cream and 1 cup of the cheese until well mixed. Stir in hash browns. Spoon into 13x9-inch baking dish sprayed with no stick cooking spray.
Mix corn flakes and melted butter in small bowl. Top potatoes with remaining 1 cup cheese and corn flake mixture.
Bake 40 to 45 minutes or until top is golden brown and bubbling around edges. Let stand 10 minutes before serving.