Peel the grapefruit with a knife so that the white skin is removed as well. Cut out the fillets. Squeeze juice out of the partitions and peel remains and collect them. Weigh out approx. 750 g fruit flesh and juice, place in a bowl with jam sugar.
Peel the bananas, weigh 250 g and cut into slices. Heat the grapefruit-sugar mixture while stirring. Let it boil for about 2 minutes at high heat while stirring. Add bananas and cinnamon, cook for another 1-2 minutes (gelling test!).
Pour hot into clean, dry glasses. Immediately close glasses tightly, turn them upside down for about 10 minutes, then turn them over and let them cool down.