Chop and prepare garnishes: finely chop the 1 bunch cilantro, then set aside a small amount of the chopped cilantro for garnish. Quarter the 6 grape tomatoes and set aside a small portion of the quarters for garnish; keep the remaining quarters for the guacamole.
Cut the 1 ripe avocado lengthwise around the pit. Twist the halves apart, remove the pit, and scoop the flesh into a medium bowl.
Mash the avocado with a fork until you reach your preferred texture (leave some small chunks for texture).
Pour the juice from the 1/4 lime over the mashed avocado and stir to combine.
Add the 1/2 small diced red onion and most of the chopped cilantro (reserve the garnish portion) to the bowl.
Add the remaining quartered grape tomatoes (not the reserved garnish pieces) to the avocado mixture and gently fold everything together until evenly combined.
Season with a pinch of salt and a pinch of black pepper, taste, and adjust by adding the same pinches again if desired.
Transfer the guacamole to a serving bowl and garnish with the reserved tomato quarters and the reserved chopped cilantro.