Season the boned pike-perch fillets with salt and pepper. Heat olive oil in a pan and fry the fillets on the skin side until the fish is almost cooked from the bottom. Heat butter and sauté the cleaned spinach. Pour in cream and season with salt, pepper and nutmeg. Cover and simmer for 1 minute. Remove from heat and puree with a hand blender. Then pass through a sieve. Fold in whipped cream before serving. Stir balsamic vinegar with olive oil, salt and sugar to make a marinade. Marinate herbs and lettuce with it. Arrange the finished spinach cream on the preheated plates. Place a pike-perch fillet on each and garnish with the marinated lettuce and herbs.