Cut the avocado; discard the stone. Cut the flesh into half-inch cubes.
Cut the tomatoes and onion into half-inch cubes as well.
Place the avocado, tomato and onion cubes in a bowl.
Pour in the olive oil and lemon. Add the salt, sugar and pepper. Toss lightly but thoroughly (using the hands works best).
Sprinkle the parsley and onion leaves on top and enjoy!