Make the dressing: 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. Whisk the garlic paste, lime juice, salt and chili powder together in a bowl. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream.
For the salad: Cook the corn, bell pepper, and onions in the olive oil until beginning to brown over medium-high heat in a skillet. Toss in the black beans and cook until warm. Add the dressing and toss to coat evenly. Adjust seasoning with salt and pepper. Remove from the heat and gently fold in the tomatoes, avocado, and cilantro. Serve.