Make sure your fish is really very fresh though, as you are just using the acidity of the marinade to "cook" the fish, rather than applying any heat to it.
I bought my mackerel whole, so had to gut, fillet and pinbone them myself. If you want you can get them pre-prepared into fillets, but try to get the fishmonger to do this to a whole fish in front of you, rather than buying ones that may have been sitting around with the flesh exposed to the air for hours, or sat in plastic. Two mackerel and one egg roll happily fed both of us; the original recipes can be found in "Japanese Home Cooking" by Harumi Kurihara.
First of all, heavily salt the fish on both sides and lay in a seive for 2 hours. This firms them up and removes excess liquid, which is essential to the texture and taste of the finished product.
Then place them in a non-metallic dish and pour over a mix of 100ml rice vinegar, a tablespoon of caster sugar and a teaspoon of shoyu (Japanese soy sauce - this is sweeter and milder than other versions, like Chinese). Leave them to marinade for half an hour, turning them every now and again.
While the fish is pickling, make your egg roll. In a jug, mix 4 tablespoons of dashi or fish stock, 50ml mirin, 4 eggs and a tablespoon of caster sugar and beat with a fork.
Lightly oil a frying pan; I would love to get a proper rectangular tamago pan but, as I don't have one, a regular pan stands in ok.
Pour a very thin layer of the egg mixture into the pan and cook until you've got a thin pancake. Lift up one edge and roll it up. Pour in another thin layer of egg, wait until it's cooked and then roll it up with the first pancake roll inside it. Keep going until all of the mixture has been used up. Put the roll into a peice of greaseproof paper and leave to cool.
Before serving up, remove the membrane from the back of the mackerel and slice it up into bite sized sashimi slices. Slice the egg roll into generous slices and chop some spring onions to go with them. A bowl of miso soup each and a dish of shoyu, for dipping, rounds the meal off nicely.