Place the beans, onion, oats, chili powder, salt, pepper and spinach in a food processor and pulse until chunky but not pureed; if it seems dry or isn't sticking together, add a little liquid a tablespoon at a time -- you want a moist but not wet mixture. Let rest in the fridge for a few minutes if time allows.
With wet hands, shape into patties; refrigerate for 15 or 20 minutes if your schedule permits.
Heat a large skillet or griddle over medium-high and add just enough oil to coat the bottom of the pan. When hot, add the patties and cook until nicely browned on one side, about 5 minutes; turn carefully and cook on the other side until firm and browned, about another 5 minutes. Serve on buns with the usual burger fixings.