Cut lengthwise around the avocado. Gently twist the two halves in opposite directions. Remove the pit and the flesh.
Place the avocado in a bowl and mash with a fork in smaller chunks. Add the lime juice. Add the diced red onions and chopped cilantro. Season with salt and pepper.
Cut the grape tomatoes into quarters and remove the seeds. (Set aside a small amount of tomato quarters.) Add to the avocado mash. Combine well.
Transfer the guacamole to serving bowls. Garnish with the leftover tomato and cilantro.