Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In large bowl, toss ham, asparagus, potatoes, onion and cheeses. Spoon into baking dish.
In same bowl, beat eggs, milk, sour cream, lemon-pepper seasoning salt and mustard with fork or wire whisk until well mixed. Pour egg mixture over potato mixture. (Baking dish will be very full.) Cover; refrigerate 8 hours or overnight.
Heat oven to 325°F. Uncover baking dish; bake 35 minutes. Meanwhile, in small bowl, toss cornflake crumbs and butter.
Sprinkle cornflake mixture over partially baked casserole. Bake uncovered 30 to 35 minutes longer or until knife inserted in center comes out clean and thermometer inserted in center reads 160°F. Remove from oven; let stand 15 minutes before serving.