This here makes a perfectly respectable lunch despite what you might think. You can try and reason as to why you shouldn’t have potatoes bathed in duck fat, but all of your thinking will fall short of one germane truth against which you will have no argument: they are delicious. These are humble ingredients, potatoes, mushrooms. The whole thing will run you about six dollars, excluding the duck fat. How’s that for economy eating?
You can purchase duck fat a nicer stores or your butcher, or if you cook duck, and you should, you should absolutely save the fat that renders out. Not doing so results in a great culinary tragedy. I actually like cooking duck precisely for the duck fat, because, to me, it’s the best part of the duck.