This is my mother-in-law's recipe for homemade ketchup. It is very time consuming but delicious!
2 gallons tomatoes
3 medium onions
2 bell peppers
Cut up the tomatoes, onions and peppers and cook until done.
Strain through sieve.
To the strained juice add the following:
3 cups sugar
1 pint vinegar
3 tsp salt
1/2 tsp Cinnamon
1/2 tsp allspice
1/4 tsp cloves
1/4 tsp mustard
Cook until it reaches the desired thickness. This can take quite a while (hours) and must be stirred frequently to avoid sticking and burning, especially towards the end of the process.
Ketchup can be poured into canning jars and processed in a hot water bath canner for 10 minutes.
Note: I often use less vinegar but do so at your own discretion. The vinegar increases the acidity of the tomatoes making them safer to can for long term storage.
Updated August 23, 2010
I just finished a modified batch of ketchup and wanted to post the changes so that I don't forget them. Rather than cook the onions and peppers with the tomatoes and then strain, I ran them through a food processor and the added them to the strained tomato juice. In addition, I used 1/2 brown sugar and half white sugar in the recipe. I think the next batch I make, I am going to experiment with using honey instead of sugar. I cooked this last batch in the slow cooker. It took almost 48 hours and still was not quite as thick as I would have liked but rather than risk burning the ketchup (very easy to do!), I added some tomato paste to thicken it up. It turned out beautiful and has a wonderful flavor.