Wash the tomatoes, cut out the stalk and cut the tomatoes into pieces. Peel and chop the onions. Boil the tomatoes, onions, sugar, mustard, vinegar, 1 level tablespoon of salt and 2 level teaspoons of pepper in a large, wide pot, stirring occasionally. Simmer covered over medium heat for about 30 minutes.
Finely puree the tomatoes with a hand blender and pass through a sieve. Put the tomato puree into a wide pot. Stir in tomato puree and bring everything to the boil. Let it boil down to ketchup for another 2 hours while stirring occasionally.
Season to taste with salt, 1 pinch of sugar and vinegar. Fill ketchup hot into prepared clean bottles (1 litre each) and close them tightly. Store in a cool and dark place. Shelf life approx. 6 months.