Wash the rocket, dab dry, cut off the stems, put something aside for garnishing. Drain tuna and capers. Drain tomatoes, collect oil. Dice tomatoes. Puree tuna, tomatoes, capers and 2 tablespoons of tomato oil.
Season to taste with pepper. Peel, wash and cut the potatoes into pieces. Cook in boiling salted water for about 20 minutes. Wash the meat and dab dry. Coat 1 piece of foil with oil, spread half of the rocket on top.
Lay 1 double fillet flat on top, season with salt and pepper. Spread with half of the tuna cream. Roll up the chicken and rocket using the foil. Process remaining ingredients in the same way.
Peel onion, chop coarsely. Heat the oil in an ovenproof pan and sauté the onion. Fry the chicken rolls lightly on each side. Place in the preheated oven (electric cooker: 175 °C/ convection oven: not suitable/ gas: level 2) with the seam facing downwards.
Pour stock over it and cook for 12-15 minutes. Heat milk and butter. Drain the potatoes, add the milk mixture and mash into puree. Season to taste with salt and nutmeg. Remove the chicken from the pan.
Pour frying stock through a sieve in a pot, bring to the boil. Stir starch with a little water until smooth, stir into the boiling liquid. Simmer for at least 1 minute. Season to taste with salt and pepper.
Cut the chicken roll into slices. Serve with sauce and puree. Garnish with set aside rocket and capers.