Peel, wash and cut the potatoes into pieces. Cover and cook in salted water for about 20 minutes.
Grate parmesan. Clean, wash and shake dry the arugula. Put something aside for garnishing, chop the rest roughly. Clean and wash the tomatoes, cut off a small lid on each. Scoop out the tomatoes with a spoon, keep the inside of the tomatoes.
Season tomatoes inside with salt and pepper.
Drain and finely mash the potatoes, stirring in the milk and 5 tbsp. oil. Fold in rocket salad and 2/3 parmesan. Season to taste with salt and nutmeg. Fill the puree into the tomatoes. Place the tomatoes in a greased casserole dish.
Sprinkle with the rest of parmesan and pine nuts and sprinkle with 1 tablespoon of oil.
Chop the inside of the tomato a little smaller. Stir in 1-2 tablespoons of oil, season with salt and pepper. Spread the mixture around the tomatoes. Bake in the preheated oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3) for about 20 minutes.
Serve, garnish with arugula.