Cut the apple and beets into cubes, removing the curved parts and reserving them for later. We cut all these cubes into sheets, each sheet into strips and each strip in small dice. It is very important that they are the same size.
We wash the branch of celery and peel it to remove their strands. We cut it into strips and, as we have done with the apple and the beets, we cut the strips into small cubes. We mix apple, beet and celery in a deep bowl.
We shred the clippings curves of the beet and the apple together with the avocado, peeled and pitted, the oil, the lemon juice, the mustard, salt and ground black pepper to taste. Mix this puree with the diced vegetables, adding it little by little until you get a creamy tartare.
It is possible that we have part of this puree left over, which we can use to spread on toast or as a cream for dipping. In any case, we keep the beet tartare in the fridge until the moment of serving, decorating the plate to taste.