Rinse the plaice fillets under running water and pat dry, then salt and roll in flour. Heat the oil in the pan over medium heat and fry the plaice fillets on both sides for a total of 3 minutes.
Heat the butter in a saucepan and use the flour to make a light roux. Add the mustard, pour in the broth and milk and stir until smooth. Finally add the capers, season with sugar, lemon juice and dill and simmer on low heat for about 5 minutes.
Serve the plaice fillets with the sauce and boiled potatoes and green salad.