For the rhubarb-tomato ketchup, rinse the rhubarb stalks with cold water and cut away any woody ends. Cut the stalks into pieces about 3 cm long. Wash and quarter the tomatoes, cutting away the stem end.
Peel and roughly chop the onions and garlic. Remove the seeds from the chili pepper and cut into fine rings.
Place all ingredients in a pot and bring to a boil. Reduce by half over low heat, uncovered (takes a good 1 hour). In the meantime, stir frequently. Strain the mixture through a hair sieve and bring to a boil again. Cook uncovered for another 15 minutes. Season to taste and pour into prepared jars or bottles while still hot. Store the rhubarb-tomato ketchup in a cool, dark place.