Peel and pit the avocados, finely chop the red onion, cilantro, and tomato (if using). Squeeze lime juice and set all aside.

In a mixing bowl, scoop out the avocado flesh and mash with a fork or potato masher until smooth but slightly chunky.

Add chopped onion, cilantro, tomato (if using), lime juice, salt, and pepper to the mashed avocado. Mix thoroughly.

Transfer guacamole to a serving bowl and arrange tortilla chips on the side for dipping.