Starch
Many recipes use wheat flour to ensure that the potatoes stick together, but I’ve found that my favorite approach is to re-employ the starch from peeled potatoes: dump them in a water bath and allow the starch to settle at the bottom, then pour off the water to use as a binder. This step takes an extra 10 minutes, but is well worth it in terms of reducing food waste (and saving money buying tons of potato starch).