Peel the peels from the green layer with a peeler.
Cut the crusts into cubes.
Pour in water and cook after boiling for 10 minutes.
Drain the water, and cover the crust with sugar and add 1 liter. water. Simmer for about 3 hours, stirring occasionally. Add zest 5 minutes before cooking. Cooking can be divided into 3 stages. After boiling, cook for 1 hour, then cool completely and repeat the same 3 more times.
Put the finished jam in sterile jars and roll it up.