
Sprinkle a pinch of salt and a teaspoon of lime juice over the sliced red onion in a small bowl. Allow it to sit for 10 minutes, then strain out any excess liquid.
Combine the softened red onion with sweetcorn, black beans, avocado cubes, and quartered cherry tomatoes in a large mixing bowl.
Add the green chilli, cumin, chopped coriander, oil, lime zest, and remaining lime juice to the bowl. Mix well and adjust the seasoning with salt and pepper, adding more chilli if you desire extra heat.
Arrange the lettuce leaves around the edge of a serving platter and mound the vegetable mixture in the centre. Serve immediately.