Using a vegetable peeler, remove the thin yellow peel, leaving the thick white pith intact. Remove and discard the inner fruit and tough membranes. Slice the pith into thin strips.

Place the pith strips in a bowl of cold water, transfer to the refrigerator, and let soak for 3 days, changing the water once a day. This helps remove the bitterness.

Drain the pith strips and transfer them to a saucepan. Cover with water, bring to a boil, and simmer for 10 minutes. Drain and repeat once or twice more if the pith still tastes bitter. Rinse the pith in cold water to cool, then gently press the strips between your palms to remove excess water. Be careful not to crush the strips. Set aside. 

Combine the sugar and 2 cups of water in a saucepan. Bring to a boil, stirring until the sugar dissolves. Continue boiling for about 8 minutes, until a light syrup forms.

Add the pith strips to the syrup. Reduce the heat and simmer gently for 45–50 minutes, stirring occasionally, until the pith becomes slightly translucent and the syrup thickens. Add the cardamom during the final 10 minutes of simmering.

Stir in the lemon juice and simmer for another 5 minutes. Remove from the heat and stir in the rosewater.

Let the preserve cool slightly, then transfer it to clean, sterilized jars, and seal. Refrigerate after opening.