Preheat the oven to 220°C/425°F/Gas 7.
Wash the mullet once more and wipe it down with kitchen paper.
Make four cuts, evenly spaced, from head to tail. Cut into where the flesh is deepest, along the back of the fish; cutting into shallow ribs will not aid the cooking.
Pick all the leaves from the parsley, wash them and make sure they are dried thoroughly in a salad spinner or on a tea towel. Chop the parsley to medium-fine and put it into a large mixing bowl. Peel and chop the onions to the finest dice you can and add these to the bowl as well. Do the same with the garlic.
Put the kettle on and get some water boiling. Cut the stem out of the tomatoes with a small knife and cut a shallow cross on the bottom. Put them in a heatproof bowl and cover them with boiling water. Count to 30 before tipping the water away. Peel the tomatoes and take out the seeds. Chop them roughly to the size of a five-pence piece with straight sides. (Sorry, I couldn't think of better terminology).Add them to the bowl. Toast the cumin seeds in a dry pan, but do not burn them. Yes, in the bowl they go. Finally, add 1 heaped teaspoon of the cayenne pepper, the olive oil and the lemon juice to the bowl. Season with salt to taste, although I advocate one more pinch on top of what you think is right. Mix everything up thoroughly.
Take two long sheets of foil, long enough so that when each is doubled over they're about 8cm longer than the fish at both ends. Doubling over the foil should prevent the bag from ripping. Lay one doubled sheet on the work surface and oil the area the fish will sit on, making sure that you really have covered the tail and head ends, as they can get gluey and stick to the foil, resulting in damage to the mullet when turned out. Salt the oiled area lightly. Put the fish on the foil. Opening up the belly cavity with one hand, pack all the charmoula into the fish, including the now-empty gill area. Pour any juice around the fish. Oil the fish on the upside and sprinkle with a little more salt. Take the top sheet of foil and lay it over the fish. Work all the way round the two sheets, folding the edges over and inwards about three times and leaving a small baggy area between the folded edges and the mullet.
Gently lay your shiny parcel in the preheated oven. It will take longer than you think to cook; I would suggest about an hour. Before you even think of checking the fish, the loose area of the bag should have puffed up with steam. To check the fish is cooked, take a thin knife and push the tip in just behind the head. This is where the flesh is deepest. It should slide in easily, with only the faintest resistance as the knife approaches the backbone.
When the time has passed, take the fish from the oven and put the bag on a suitable serving dish, one with sides, as you will find a lot of tasty juice within. Unwrap the bag, being careful not to let your hands get caught by the steam. If you feel you can slide fishy from the foil on to the plate, do so, otherwise serve it from the open foil. Make sure everyone gets some of the fish and stuffing along with some juice. It goes well served with steamed couscous.