Peel, wash and halve small potatoes. Peel and slice the carrots. Clean and wash the celery and cut it into diagonal pieces. Clean, wash and cut spring onions into rings.
Clean, wash and cut the tomatoes into pieces. Heat oil in a pot. Briefly sauté potatoes, carrots, celery and spring onions in it. Add stock, bring to the boil and cook for 10 minutes.
Peel and wash the large potato and grate it into the soup. Let it cook for another 5 minutes. Add the tomatoes shortly before the end of the cooking time. In the meantime, wash the parsley and thyme and put them in the blender.
Chop the parmesan, peel the garlic and put it in the blender with the oil. Finely chop everything. Season vegetable soup with salt and pepper. Add capers to the soup. Serve the vegetable soup with pesto and sour cream.