Chilli beef and bean burgers

Name: Chilli beef and bean burgers
Cuisine: American
Ingredients: 1 small onion, quartered; Small can (about 210g) red kidney beans, drained and rinsed; 250g lean beef mince; Wholemeal breadcrumbs (from 1 slice of bread); 2 tsp mild chilli powder, or to taste; 1 egg, beaten; 1 tbsp tomato purée; 1 extra large tomato, sliced; 1 red onion, sliced; Lettuce leaves; 4 wholemeal rolls