I used 2 400 g canned of diced tomatoes. Most tomato cans sold at the supermarket are 400g.
These are all the ingredients you need.
Use a pot that withstands acid, like an enamel pot. Put in the tomatoes, sugar (80-90% of the amount indicated) and lemon juice in the pot, mix and start cooking over medium heat.
When it comes to a boil, scum will rise to the surface, so scoop off diligently. When most of the scum is removed, lower the heat a bit and simmer for about 30 minutes while stirring occasionally.
Sterilize storage jars in the meantime. Put the jars in a large pot of water. Boil the jars for about 5 minutes. Take out, drain and leave to dry.
After 30 minutes, it will have reduced quite a bit. Taste now and adjust the sweetness. If you think it's not sweet enough, add the rest of the sugar.
Simmer for another 5 minutes and it's done. Simmer to the level you like.
Pour the jam into the sterilized jars while it's still piping hot. Fill the jars up to the rim. The jars and jam are both very hot, so be careful.
As soon as the jars are filled, turn upside down and leave them as is. The heat of the tomato jam will disinfect the lids. When the jars have cooled, the lids will have popped inwards.
It looks like this when it's done.
Story Behind this Recipe
Tomatoes are packed with umami. Canned tomatoes are picked when they are at their best and ripest. You can stock them year-round, and you don't need a cutting board and knives either. You don't even have to measure it out. I thought there's nothing as convenient as this, and so I created this recipe.
Simmer until the volume is reduced to about 60%. If you want to keep the jam for a long time, add extra sugar. As you simmer and reduce the tomatoes, they will sink to the bottom of the pot. It burns easily, so near the end of the simmering process keep on stirring up the pot from the bottom.