Home-made kunserva (tomato puree/concentrate)

Wash and roughly chop about six kilos tomatoes;  remove all seeds and as much liquid as possible;  put in a large pan with half a cup olive oil, 3 tbs rock salt and 2 tbs sugar and cook for about 15 minutes, cool. Process the tomatoes, skin and all, then transfer to a blender and blend till smooth. Lightly grease a large dish and pout the tomato mix, sprinkle some more sugar and rock salt.  Bake in a hot oven (fan 250) for about 1.5 hours or till the mixture thickens, is reduced by half and darkens in colour.  Store in a sterilised jar and top with some olive oil, rock salt and crushed black pepper.