Cut them in half vertically and slice them thinly with zest. Seeds should't be taken off.
It will be like this. I recommend using a enamel cast iron pot that resistant to acidic foods.
Add measured sugar and lemon juice and place over low heat. Stir it gently until it comes to a boil. Remove seeds and skim off the scum.
Simmer for 20 minutes after starting boiling. Adjust the simmering time depends on the season.
Transfer to a jar that has been sterilized in boiling water, and it's done.
Story Behind this Recipe
The Sudachi tree in my garden produces more and more fruit each year, And I got about 300 fruit this year. I made this Sudachi confiture using about 50 fruits.
Confiture is softer than jam. In this case, I used sugar equal to the weight of 40% of the sudachi because I used ripe sudachi. If you want to make a jam, use sugar equal to the weight of 50% of the sudachi. Taste it and adjust the amount of sugar. You can use the whole fruit including zest, fruit and juice.