Preheat a grill to medium high. Brush the corn and onion rings with olive oil and season with salt and pepper. Grill, turning occasionally, until lightly charred and tender, about 8 minutes for the onion rings and about 10 minutes for the corn. Set aside to cool.
Combine the lime juice and ¾ cup cilantro in a blender and pulse until chunky. With the blender running, pour in the olive oil in a slow, steady stream to make a thick dressing. Season with salt and pepper.
Cut the corn off the cobs and roughly chop the onion. Combine in a large bowl with the avocados, tomatoes, beans and pimientos. Roughly chop the remaining ¾ cup cilantro and add to the bowl. Drizzle with the dressing and toss well. Season with salt and pepper, if needed.