Preheat the oven to 200C/180C Fan/Gas 6. Place the plantains in a roasting tin and roast for 15 minutes until softened. Leave to cool slightly, then peel and mash with a fork in a large bowl. Add the black beans, salt, black pepper, chilli, mixed herbs, coriander and onion. Mix well then stir in the flour to form a thick, sticky mixture.
Divide the mixture into four and form into burger shapes. Cover and place in the fridge for 1 hour (this helps them hold their shape when cooking).
Heat the oil in a griddle pan and cook the burgers for 7 minutes on each side until golden brown.
To serve, add the lettuce to the bun bases, top with tomato slices and the burgers. Finish with avocado and sweet chilli sauce. Top with the bun lids and serve.
For the burgers
2 ripe plantains
400g tin black beans, drained and rinsed
1 tsp sea salt flakes
1 tsp freshly ground black pepper
1 Scotch bonnet or other chilli, seeds removed, chopped