Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the sweet potatoes. Chop into chunky fries. Spread out on a baking tray. Drizzle with a little olive oil. Season with salt and pepper. Bake in the oven for 25 mins till tender and crisp at the edges
Peel and finely chop the shallots. Finely grate the zest from the lime. Juice it. Halve the chilli. Flick out the seeds and white bits for a milder flavour. Finely chop it. Peel and grate or crush the garlic.
Rinse and drain the black beans. Pop in a food processor with the shallots, egg, breadcrumbs, half the lime zest, half the chilli and the garlic. Season. Blitz till smoothish. If you don't have a processor, mash the beans in a bowl and mix with the other ingredients.
Shape the mixture into 4 flat burgers. Pop on a plate. Cover with cling film. Place in the freezer to chill for 10 mins.
Peel and chop the blood oranges into small chunks. Peel and finely chop the red onion. Mix the blood orange, red onion, half the lime juice, the remaining lime zest and chilli together in a bowl to make a salsa.
Tear the leaves off half the lettuce. Rinse. Dry in a salad spinner or with kitchen paper. Pop in a bowl. Add the rest of the lime juice, a splash of olive oil and some salt and pepper.
Heat a drizzle of olive oil in a frying pan. Fry the bean burgers for 2-3 mins each side till browned and crisp.
Serve the bean burgers with the sweet potato fries, blood orange salsa and lettuce on the side.