Heat a knob of butter and a little oil in a pan. Cook 1 finely sliced onion and 1 sliced garlic clove until soft, stirring regularly. Transfer to a plate.
Slice 2 × 350g skinned gurnard fillets and then dust with seasoned flour.
Return the pan to the heat and melt another knob of butter with a little oil. Add the fish, skinned side down. Cook for about 2 minutes, until it is nicely browned on one side. Return the onion and garlic to the pan.
Add 75g peas, pour over 200ml dry cider and simmer everything gently for 4 minutes. Divide the fish between 2 warmed plates.
Return the pan to the heat. stir in 2 tbsp chopped parsley, and 1 tbsp chopped mint followed by a knob of butter. Bubble until thick and glossy. Spoon the peas and sauce around the fish and serve with crusty bread.