First prepare the dough for the catfish by mixing the flour with the egg yolks, the beer and the melted butter or vegetable oil and a pinch of salt to a smooth dough and let it rest for at least 1/2 hour.
In the meantime, wash the fillets of catfish, dab dry with kitchen paper, skin and carefully debone with kitchen tweezers. Season both sides with salt, sprinkle with a few drops of lemon juice and roll in flour.
Now whip the egg whites into snow and carefully fold it into the yolk mixture. Heat enough lard in a large frying pan. Pull the catfish fillets one after the other through the beer batter and fry them in the hot lard like escalopes until golden brown on both sides.
Lift out the finished fillets, drain well or dab with a paper towel.
Garnish the catfish with lemon wedges and serve. Serve with parsley potatoes and Vogerlsalat.