Melt half of the butter in a saucepan and stir in the mustard and egg yolks. Allow to thicken over low heat without the eggs curdling. Then stir in the yogurt. Season to taste with sugar, salt and pepper.
Cut the fish into pieces about 5 cm in size, acidify with lemon juice, season with salt and pepper. Melt the remaining butter in a pan and fry the fish pieces for 2 minutes on each side.
Put the sauce on preheated plates and arrange the pieces of fish on top. This goes well with leaf spinach or steamed leeks and potatoes.