Drain the black beans really well, then put in a large bowl. Mash them roughly – some whole beans should remain – and then season generously.
Finely slice the spring onions and add three-quarters of them to the bowl, reserving the rest for later. Stir in 2 tbsp of the salsa. Add a sprinkling of plain flour if the mixture seems a bit on the wet side.
Shape into two rounds about the size of tennis balls. Flatten with the palm of your hand, so that you have two patties about 3cm thick. Chill in the fridge for at least 30 minutes.
Heat 1 tbsp vegetable oil in a frying pan over a medium heat. Add the burgers and cook for 5 minutes on each side. Remove from the pan.
Slice the burger buns in half and place cut-side down in the pan. Toast for 1 minute until lightly charred. Put a burger inside each bun. Top with another spoonful of salsa, scatter over the remaining spring onions and serve straightaway.