If your tuna in oil is not top quality (the French brand "Le Connétable" is a good one I recommend, even if a bit pricey), don't use the oil from the tin, stick with olive oil.
The quantity of olive oil to add in #stage 3
is for guidance, so stop as soon as it reaches the desired consistency.
In French, potatoes baked in their skins are described as "en robe des champs" (in their field gowns - a quaint way of saying "as they come from the field"). A classic way French kids deform this is to say that the potatoes are "en robe de chambre" (in their dressing gowns). This worked for me and my sisters when we were small, it worked for my boys; I wouldn't be surprised if it works one day for my grandchildren. I hope you like it too.