Wash the pikeperch fillets, drizzle with lemon juice and set aside.
Put the fried potatoes in a pan, season and fry over medium heat.
Clean and slice the mushrooms, peel the onions and cut into rings.
Dab the fish with a paper towel and cut into portions, season with salt and pepper. Put the flour on a plate and roll the pieces of fish in it, knocking off any excess flour. Heat 2 tablespoon clarified butter in a pan and fry the pikeperch on both sides until golden brown, keep warm.
Put 1 tablespoon of clarified butter in the pan and fry the mushrooms until golden brown. In a second pan, heat 2 tablespoons of clarified butter and fry the onion rings until crispy.
Spread pikeperch on plates and top with mushrooms and onions, serve with the fried potatoes.