Wash the meat, dab dry and cut into slices about 1 cm thick. Tap the neck steaks a little flatter. Peel onion, cut into fine rings or slice.
Line a box form (approx. 30 cm long; approx. 2.5 l capacity) with baking paper. Cut four strips of approx. 1 m length from the kitchen yarn. Place the ribbons lengthwise in the box mould and let each ribbon protrude evenly over the edge. Sprinkle each slice of meat with pepper and coat with some mustard. Cover with 1 slice of ham and 1 slice of cheese, fold up.
Place the slices of meat with the open side facing upwards and the onion rings in the tin (set the tin slightly at an angle so that the ingredients for the sandwich roast can be layered in easily). Season each layer with a little salt. Tie the meat firmly with kitchen string. Roast in the preheated oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer) for approx. 2 hours. Cover with baking paper after about 1 hour.
Wash the tomatoes, cut them in half. Wash and chop the rosemary. Remove the roast, carefully turn it over on a baking tray and remove the baking paper. Leave to rest for a short time and turn them upside down again with two spatulas. Spread tomatoes around the roast. Sprinkle with rosemary and sea salt.
Mix paprika and oil. Coat the roast all around with it. Roast in the hot oven for about 30 minutes. Remove kitchen string. Arrange everything. Serve with a green salad.