Make the black bean mixture

• Mash the black beans in a bowl until mostly broken down but still a bit chunky.
• Add the egg, bread crumbs and a generous amount of seasoning, since black beans are bland.
• Mix until the mixture holds together, then shape into small thin patties (two per burger).

Slice the onion and heat the pan

• Slice the onion very thinly.
• Heat a cast iron skillet over medium-high heat with a little oil.

Smash and cook the patties

• Spread a layer of sliced onion in the pan and place a thin patty on top, pressing it down with your hands or a flat tool.
• Cook for a couple of minutes, then flip.
• Repeat with the remaining onion and patties for a double-decker burger.

Toast the buns and melt the cheese

• Add the buns to the pan to toast.
• Top the patties with cheese if using, add a splash of water and cover to steam so the cheese melts and the buns soften.

Assemble and serve

• Stack the two patties with onion and cheese in the toasted bun.
• Serve immediately.