Peel and chop the onion. Peel, wash and cut carrots into small pieces. Heat the fat in a pot. Fry onion and carrots in it. Add stock and cook covered for about 10 minutes.
Add the strained tomatoes. Puree with the cutting stick of the hand mixer. Stir in cream and season with salt and pepper. Peel potatoes and cut into small cubes. Heat herb butter in a pan and fry the potatoes in it for 3-4 minutes at medium heat until golden brown.
Drain the potato croutons on kitchen paper. Heat soup again briefly and fill into bowls. Serve sprinkled with croutons. Garnish as desired with chopped parsley and a parsley leaf.