Grate the rind of one of the oranges and stir into the soured cream in a bowl.
Peel the other orange, then cut the skin and white membrane off both oranges and the grapefruit. Working over a separate bowl to catch the juice, cut between the membranes to remove the segments. Mix the fruit segments and juice with the cinnamon and sugar in a small saucepan. Heat over a low heat for 1–2 minutes.
Meanwhile, put the croissants on a baking sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 5 minutes, or until thoroughly heated and slightly toasted.
Split the toasted croissants lengthways and spoon the fruit mixture over the bottom halves. Top with a spoonful of the soured cream mixture and a sprinkling of cinnamon, and replace the top halves. Serve immediately.