Dice them up. (This one included some onion rings, but that works just fine too.)
Parboil a few potatoes and dice them up too. Put everything in a skillet with a few tablespoons of butter. If your doggy bag didn't include onion rings, you could saute half a diced onion first to add a little flavor.
Fry for a good long while, scraping up all the bits on the bottom of the pan. The soggy leftover french fries are important but the crispy bits at the bottom of the pan are critical.
I said it wasn't healthy. But it is absolutely delicious, especially with farm-fresh egg omelets.