Cut open avocado lengthwise cutting around the seed. Separate the two sides. With your knife, strike the seed with the blade and pull the seed out. Hollow out the two avocado halves with a spoon and place into a small mixing bowl. With a potato masher or fork, mash avocado. Dice up the onion and tomato to your liking. (I like them diced very finely.) Mix into avocado. Squeeze lime into mixture. Add salt and pepper to taste. For best results, allow to sit 10 minutes before serving. Serve alongside tortilla chips.