Wash the orange, finely chop (without the white part), peel, cut in half and squeeze out the juice.
Cut the zest into thin strips, squeeze the juice from half a lemon, prepare the remaining ingredients.
In a saucepan, mix: orange juice and lemon, sugar, zest and half the required amount of water. Bring to a boil, reduce the heat and cook over medium heat for 5-7 minutes.
Dilute the starch in the remaining water and, stirring, pour into a saucepan, stir, remove from heat.