Once the potatoes and kohlrabi are cooked (about 30 minutes later), remove from water and place in a large mixing bowl along with the onion and sundried tomatoes. Keep some of the water - you might need to add it later on to smoothen the texture of the soup. Blend until homogeneous. Add salt and pepper to taste, and some of the water from the boiled veggies if the soup is too thick and clumpy. Mix again until smooth. Serve with chopped parsley and grated fresh kohlrabi scattered on top (the latter for a good contrasting textural element). Lastly, decorate with basil flowers, if desired.