Adjust oven temperature to 375 F. Coat a 9 X 13-inch baking pan with oil or butter; layer the potato, then corned beef, shallots & Brussels sprouts. Repeat again using all the ingredients. Whisk the flour into the cooled milk/butter combo. Pour sauce over all; sprinkle with garlic powder & top with Parmesan cheese. Cover with foil & bake for 30 minutes. Remove from oven; cool for a few minutes & serve.