For the salad:
1) Toss together the beans, corn, bell pepper, and onions. Add the dressing and toss to coat evenly. Gently fold in the tomatoes, avocado, and coriander.
2) Adjust seasoning with salt and pepper, to taste, and serve.
For the dressing:
1) Smash the garlic clove, sprinkle with a pinch of the salt, and, with the flat side of a large knife, mash and smear the mixture to a coarse paste.
2) Whisk the garlic paste, lime juice, remaining salt and chilli powder together in a bowl.
3) Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream.