Drain and rinse the beans well and add to a mixing bowl. Using a masher or fork, mash the beans until they are not quite smashed.
400 g black beans
Add the breadcrumbs, egg, coriander, chilli powder, smoked paprika and a pinch of salt. Mix well until they bind together.
50 g breadcrumbs, 1 tsp smoked paprika, 1 tsp chilli powder, 1 large eggs, ¼ bunch coriander
Using your hands divide into two portions and mould into pattie shapes.
Place the patties into a fridge to firm up.
Place a second frying pan over a high heat and add a drop of oil. When hot add the onions and fry until softened and translucent. Remove from the pan and put to one side keep warm.
2 tbsp olive oil, 1 medium onion
Place the olive oil into a frying pan over a medium heat. Fry the burgers for 3 minutes and flip over until the good crust has formed and cooked through.
While the burgers are cooking. Add the mushrooms to the second pan with a little oil and butter with a small pinch of salt and fry until soft.
6 large chestnut mushrooms
Slice the rolls in half and lightly toast.
2 large baps
When the burgers is cooked, add the cheese slices and cover with a loose piece of foil so the steam melts the cheese.
When the rolls are toasted spoon the mayonnaise over the bottom half and then place the burger, onions, mushrooms, lettuce, sliced gherkins and a small spoon of salsa.