Not all leftovers are created equal. Turkey is nearly equally valuable three days later as it is on Thanksgiving day. Green beans, not so much.
If you are trying to eat nutritiously and sustainably, lentils have a lot going for them. They are hardy and do well in varying climates. They are legumes, so they deliver plenty of protein and they fix their own nitrogen. These burgers help cut synthetic fertilizer use and encourage diverse rotations.