1. In a bowl mix together the breadcrumbs, lemon rind and juice and chives with salt and pepper and bind the mixture with the beaten egg.
2. Divide the stuffing between the fish pressing well into the cavity and securing the opening with a wooden cocktail stick.
3. In the cooker heat the butter and oil and quickly brown the fish on both sides, lift out , dram off the butter and oil and put water.
4. Grease the trivet with a little butter or margarine and place into the cooker.
5. Lay the fish, head to tail on the trivet, close the cooker, bring to high pressure and cook for 5 minutes.
6. Reduce the pressure quickly and remove the cocktail sticks.
7. Garnish the mackerel with chopped chives and parsley sprigs, if desired and serve with wedges of lemon.