Wash the kumquats carefully and rinse them. With a sharp knife cut the hard tip whith the stem one by one. Slice the kumquats into very thin rounds. As you slice, remove the seeds with the tip of your knife and reserve them. Put the slices in big bowl.
Peel and juice the lemon. Add both peel and juice to the sliced komqats.
Put the reserved kumquat seeds in a strainer bag (cheese cloth or similar) and place them in the bowl. Pour the water, cover and set aside overnight.
The next day put the sugar in a wide pot and pour the kumquats, the seeds bag, and their soaking liquid over the sugar. Heat the pot very slowly over medium-low to dissolve the sugar, about 10 minutes. When it begins to bubble, raise the heat to medium-high and bring to a boil. Cook without stirring for 15 minutes until the marmalade looks glossy. Stir the marmellata gently. Turn off the heat and let stand for 15 minutes. Discard the seeds bag and the lemon peel. Ladle the marmalade into sterilized jars and let them sit and cool to room temperature. Refrigerate for up to 2 months.
Recipe by The Quotidian Cook at https://www.thequotidiancook.com/en/pioneer-kumquat-marmalade/