Heat the oil in a small frying pan, add the onion and chilli and cook over a medium heat for 2–3 minutes until softened. Stir in the spice mix and cook, stirring, for 1 minute. Leave to cool slightly.
Mash the beans in a large bowl with a potato masher or fork, then add the breadcrumbs and coriander and season well with salt and pepper.
Beat the egg with the paste in a jug, then add to the bean mixture and mix together well with a fork.
Divide the bean mixture into 4 and shape each portion into a burger.
Place the burgers on a nonstick baking sheet and cook under a preheated medium-high grill for 4–5 minutes on each side until golden and cooked through.
Split the buns in half horizontally. Top the bottom halves of the buns with some lettuce and a spoonful of salsa. Place the burgers on top and finish with a spoonful of guacamole. Cover with the top halves of the buns.