Soak kumquats in warm water with baking soda for 10 minutes, then massage the skin by hand to clean thoroughly. Rinse, dry, and pat clean with a cloth.
Make four small holes on each kumquat, ensuring they are not deep to preserve natural flavor and aroma.
Place kumquats in boiling water for 30 minutes, drain, and soak again in boiling water for another 30 minutes (total 1 hour).
During soaking, gently stir occasionally with a slotted spoon to avoid mushiness, as this step removes bitterness.
In a pot, combine sugar and boiling water in a 1:1 ratio, add cardamom and cinnamon stick, and heat until sugar dissolves and syrup simmers.
Drain kumquats, slice into rings, remove seeds, and gently add to the simmering syrup.
Cook on medium heat for about 55 minutes, skimming foam to maintain clarity.
Turn off heat and stir in fresh lemon juice for glossiness and preservation.
Soak gelatin sheet in cold water for 3 minutes, squeeze out excess water, and dissolve in the warm jam with heat off.
Let the jam cool at room temperature for a few hours, then transfer to a clean jar and store in the refrigerator.