Defrost the spinach. Peel onions, dice 2 of them finely. Peel the garlic and press it through a garlic press. Heat the fat in a pan, fry the diced onions and garlic in it.
Mix with the spinach. Season with salt and pepper. Stir in pine nuts. Season the meat with salt and pepper, spread 2/3 of the spinach mixture on top and roll up or fold the meat.
Tie together with kitchen string. Place on the fat pan of the oven or in a frying pan. Bake in the preheated oven at (electric oven: 200 °C/ gas: level 3) for 1 1/2-1 3/4 hours. In between, gradually add approx. 1/8 litre of water.
Sprinkle meat with roast stock from time to time. Whip the lamb roll in aluminium foil and let it rest for about 10 minutes. Pour frying stock through a sieve into a pot. Puree the rest of the spinach and add with the cream.
Bring to the boil, season to taste with salt and pepper and let everything boil a little. Clean, wash and slice the tomatoes. Mix vinegar, salt, pepper and sugar. Stir in oil. Pour over the tomatoes and sprinkle with remaining finely chopped onion.
Cut the meat open and arrange on a plate with the tomatoes. Garnish with basil. Serve with extra sauce. Potato gratin tastes good as a side dish.