Place the stockpot on medium heat. Add into the pot the olive oil and butter.
When the butter begins to sizzle, add in all the carrots, celery and onion pieces.
Add in 1 tbsp of salt and a few grinds of the peper mill. Give a stir and put the lid on the pot to help sweat the vegetables. Stir after 5 minutes. You want to allow the vegetables to cook down and get a little color.
Add in the chopped chocolate mint and give a stir. Let the vegetables cook for another 5 minutes. If the heat is to high, turn it down.
Add the cut potatoes and the 7 cups of cold water. Bring to a boil and turn down to a simmer. Let cook for 40 minutes or until the vegetables are easily smashed with a fork.
Let the soup cool for 10 minutes and puree either with a wand or in a food processor. If while pureeing you feel the soup is too thick you can add 1/2 a cup of water at a time. You want a nice consistency so the yogurt will rest on top of the soup. You do not want a porridge but more like a light custard that is pourable. Taste the soup for seasoning.
I like to serve this soup at room temperature with a few dollops of either a Greek yogurt or Labneh as a garnish. Toasted Nan or pita bread make a nice accompaniment along with some black olive tapenade on the side.
If you cannot get chocolate mint, you could substitute any type of mint. Most garden shops have a variety of mint that is easily grown in a large pot. I have pineapple and chocolate in my garden this summer. Most grocery stores have the regular mint but the chocolate gives it an additional depth which is a great taste in the summer.
I like to peel the celery because I find it is a little stringy and by just lightly peeling the outer layer of the celery you do not get the stringy bits.