Put the onion into a bowl and pour over the lime juice. Massage together using your fingers to soften. Add the avocado and toss to coat in the lime juice, then add the pepper, coriander and chilli, if using. Toss in the beans and divide between two bowls or lunchboxes.
Scatter over the pineapple and drizzle with a little olive oil before serving, if you like. Best served straightaway.
