Rinse the quinoa under cold water for a couple of minutes.
Boil the quinoa over medium heat for about 15 minutes.
In the meantime, clean and chop the onion, the cucumber, the tomato, the green bell pepper, and the parsley. Place all these ingredients in a big serving bowl.
Once the quinoa is cooked, let it cool for at least 5 minutes before mixing it with the rest of the ingredients.
Pour the lemon juice and the olive oil in the bowl. Add the salt and pepper, and toss it all until well combined.
Let it rest in the fridge for an hour.
Cut the avocados in halves. Remove the pit and the skin gently with a small spoon.
Serve one portion of the quinoa mixture in a dish. Place one half of the avocado on top, and fill it with a little bit more of the quinoa salad. Finally, decorate it with a sprig of parsley and pepper.