Combine beans, onion, oats, chili powder and lime juice in a food processor.
Process the mixtrue until chuncky.
If the mixture doesn't stick together, add saved bean liquid. It should be moist but not wet.
Wet hands and mould the mixture into patties that are approximately 3½ inches.
In a nonstick skillet, cook burgers on medium-high heat until they are lightly browned and cooked.
Serve with letttuce leaves as the bun, and add tomato slices