1. Use most of the butter to grease 4 pieces of foil, each large enough to enclose one mackerel. Set the oven at 200°C/400°F/gas 6. Pat the fish dry. Fill the cavities with the mushroom stuffing.
2. Lay a fish on each piece of foil and dot with the remaining butter. Sprinkle lightly with salt, pepper and lemon juice and bring up the foil to make 4 loose parcels.
3. Support the parcels on a baking sheet and bake for 20 minutes. Open one parcel and test the fish with a fine skewer. It should be tender all the way through. Serve the mackerel in the parcels, each opened just enough to show the tops of the fish. Place two grilled/broiled mushrooms on each,