Rinse the tomatoes and cut a cross on the bottom with a small knife, so as to skin them easily after.
Bring a pot of water to a boil and, as soon as it boils, remove from the heat and plunge the tomatoes in the water. Leave the tomatoes in the hot water for five minutes, then drain them and immediately plunge them in a bowl of iced water.
Peel the tomatoes, remove the seeds and collect them in a colander. Let them drain for at least half an hour, so that they lose most of their water.
Now weigh the tomato pulp and collect it in a pot. I got 600 g of tomato pulp. From here you can calculate the amount of sugar needed, which will be half of the tomato pulp weight.
Cook the tomato pulp for about 10 minutes over medium heat, then add the sugar, the apple cider vinegar, the cumin powder and the chili pepper, without seeds, cut into strips.
Stir and cook for about 20 minutes, or until the jam is firm and shiny.
Purée the jam with an immersion blender and test with a saucer that you had previously held in the freezer for a few hours. If the jam is thick and slides slowly, it is ready, otherwise bring it back on the stove for a few more minutes.
Pour the jam into two sterilized jars and close them well with a lid.
If you are not using the jam straight away, put the jars in a large pot, cover with water and bring the pot to a boil. Simmer the water for 20 minutes, then turn off the heat and let the jars cool down there. You can keep the jars for several months in the pantry.