Wash the meat, dab dry and tap flatter. Peel onion and cut or slice into fine rings.
Cut 4 sections of approx. 1 m length each from the kitchen yarn. Place the yarn sections lengthwise in a box form (approx. 2.5 litres capacity, 30 cm long and 9 cm high) so that they reach evenly over the edge on both sides. Sprinkle each slice of meat with pepper, spread some mustard on it. Cover each slice with a slice of cheese and ham, fold up.
Place slices of meat in the mould with the open side facing upwards. Season each layer with salt. Tie the ends of the kitchen string tightly together over the meat. Roast in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 2 hours. Cover after about 1 hour.
Tomatoes wash, clean, about 2/3 halve. Wash the rosemary, dab dry, put something aside for garnishing. Remove the remaining rosemary needles and chop. Remove the meat from the oven, carefully slide out of the tin onto a baking tray. Let it rest for a short time and turn it over again with 2 spatulas. Spread the tomatoes on the baking tray. Sprinkle with rosemary and sea salt.
Mix paprika and oil. Coat the roast all around with it. Bake in the oven at the same temperature for about 30 minutes. Remove kitchen string, pluck meat pockets in portions. Serve with the tomatoes. Serve with a green salad.