Cut the potatoes and bring to the boil, then add the spinach. Add salt and cook until ready. Then blend with vinegar and season to taste. For the gelled cheese: heated the milk and the feta cheese, while melt the feta cheese. Add the agar and boil for 5-10 minutes, stirring continuously. Get out of the fire and add yogurt. Pour into containers and allowed to harden in the fridge - I use a container with a diameter of 11.5 cm and 2 cm. After being cooled is cut into pieces. In a pan put the sliced ​​celery and cashews, add sugar until melted and add the balsamic vinegar until caramelized. Then pour it on the kitchen aluminum foil to cool. Serve the soup with the chopped aspic feta cheese, caramelized celery and cashews and a few drops of olive oil.