Bring a small pot of water to a boil over high heat. Using a big spoon to carefully put the egg in the boiling water. Cover with a lid. Reduce the heat to low, cook for 11 minutes. Transfer the egg to a medium bowl of iced water. Let it sit for 15 minutes. Crack and remove the egg peel under running water. Cut the egg into small dices.
In the meantime, cut the avocado in half. Remove the seed. Place half of the avocado on a chopping board flesh side up, using a paring knife, score the inside of the avocado into cubes. Scoop out the flesh. Put into a medium bowl.
Drizzle the lemon juice over the avocado. Add salt and white onion. Toss together with a spatula or spoon. Don’t overmix. The guacamole should be creamy, but with small chunks of avocado visible. Adjust with more salt or lemon juice if needed. Add the diced egg and gently mix into the guacamole. Transfer to a serving bowl and serve.