Peel oranges and grapefruits, leaving the white skin on. Cut the fruit into small pieces. Boil the fruits in 1 litre of water for about 20 minutes, covered and soft. Wash kumquats, cut them into thin slices and remove seeds.
Coarsely mash oranges and grapefruit and pass through a strainer mill (Flotte Lotte). Pour the collected liquid into a measuring cup, if necessary fill up to 800 ml with water.
Put the juice, kumquats and sugar in a wide high pot and simmer for about 5 minutes.
Wash the thyme, shake dry and pluck the leaves roughly. Add the thyme to the jelly. Pour the jelly into clean, hot rinsed sealable glasses and close them immediately. Turn glasses upside down for about 10 minutes, then turn them over and let them cool down.