Scale, gut and clean the fish and cut off the fins and gills. Melt half the butter in a pan and gently fry the onion until golden. Add the stock, vinegar, anchovy essence, bay leaf, bouquet of herbs and clove. Simmer for 10 minutes, then add the perch and cook gently for 10 minutes. Keep the fish warm.
Melt the remaining butter, add the flour and cook for 2 to 3 minutes. Gradually stir