For the dough, bring 1/4 litre of water, fat and salt to the boil in a saucepan and remove from the heat. Add flour at once and stir in. Put the pot back on the heat and stir until the dough comes off the bottom of the pot as a lump.
Remove the pot from the heat. Stir in the eggs bit by bit and fill the dough into a piping bag with star-shaped spout. Spray six rings (seven centimetres in diameter) onto a baking tray lined with baking paper.
Bake in the preheated oven (electric oven: 200°C/ gas: level 3) for 30 minutes and cut open immediately with scissors. Prepare the remaining rings in the same way. For the filling, wash a lemon and grate the peel.
Squeeze all lemons, fill up to 1/2 litre with water and bring to the boil with sugar and fat. Stir cornflour and a little water until smooth and stir into the lemon cream. Swell for five minutes at low heat and let it cool down a bit.
Fold in the egg yolk and allow to cool completely. Whip cream until stiff and fold in carefully. Pour cream into a piping bag with star-shaped spout and squirt onto the lower halves of the ring. Place the upper ring half on top and decorate with icing sugar and ground pistachios.