Peel, wash and slice the potatoes and carrots. Clean, wash and cut leek into rings. Fry the potato and carrot slices in 2 tablespoons of hot oil for about 10 minutes. Add the leek 5 minutes before the end of the cooking time and fry. Rinse fish cold and pat dry. Cut into cubes and sprinkle with lemon juice. Fry all around in 2 tablespoons of hot oil for about 5 minutes until golden brown.
Season with salt and pepper. Add the vegetable stock and cream and bring to the boil. Stir in mustard and sauce thickener. Season to taste with salt and pepper. Wash tarragon, dab dry and chop finely, except for a stem for garnishing. Add to the vegetables with the fish and heat briefly. Arrange the panfish on plates and garnish with tarragon