Line the troughs of a muffin tin with paper baking cups. Peel the orange so that the white skin is completely removed. Remove the fillets with a sharp knife between the parting skins and cut them into small pieces. Press the juice out of the parting skins
Mix flour, baking powder and sugar in a bowl. Whisk egg and oil, add orange pieces and juice. Add to the flour mixture, stir briefly. Spread the mixture into the wells and bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 20 minutes. Remove the muffins and let them cool down
For the icing, mix icing sugar with egg white and lemon juice. Depending on the consistency of the icing, add some more icing sugar or lemon juice
Put some icing on the muffins. Chop the pistachios and sprinkle them on the moist icing, allow to dry