Cut each mackerel piece in half perpendicular to the backbone (note 1). Make a cross incision (x not +, see step-by-step photo) on the skin side of each piece. This is to cook the fish evenly and let the flavour penetrate into the flesh better.
Add about 3 cups of water (not in ingredients) to a saucepan or a deep frying pan and bring it to a boil over high heat. Put each mackerel piece gently into the boiling water (note 3). As soon as the surface of the fish turns to white, transfer the fish to the iced water to cool down. Then pat dry with kitchen paper.
Take 2 tablespoons of water from the Simmering Sauce ingredients and mix with the miso in a bowl so that the miso becomes slightly watery (easy to dilute).
Add shallots, ginger and the Simmering Sauce ingredients, excluding miso, to a shallow pot or a frying pan that can fit all the fish pieces snugly without overlapping. Bring it to a boil over high heat.
Reduce the heat to medium and remove the drop lid. Add the miso to the pot/pan in swirling motion so that the miso spreads evenly.
Cook for 5 minutes or until the sauce thickens slightly. Do not condense the sauce too much as the sauce will become too salty (note 5). Use a ladle and pour the sauce over the fish few times while cooking.
1. If your fillet is much larger, reduce the number of sides and cut each side into 3 or 4 pieces.
2. It is best to use the white part of shallots. After julienned finely either vertically or diagonally, put them in a bowl of iced water. Each strand will start curling. The finer the julienned pieces, the curlier they get. (Looking at the photo of my dish, I didn't do a good job there...)
3. Some recipes call for pouring boiling water over the mackerel pieces to sear it. I find that this method is rather fiddly in turning over each piece to sear the other side. But if you can sear without breaking the fillets, you can try this method.
4. A drop lid is called 'otoshibuta' (落し蓋) in Japanese. It is a round lid that is slightly smaller than the opening of a saucepan. It is traditionally made of wood but I have a stainless-steel lid. It is placed on top of the ingredients in a pot to ensure the heat is evenly distributed, ingredients cook faster, and stay in place without breaking apart. It also stops the liquid from evaporating quickly.
If you don’t have a drop lid, you can make one with aluminium foil. Cut a square in foil, fold the edges to make it a round shape with the diameter slightly smaller than the pot. Then poke the foil with a knife or a chopstick to make holes in several places.
5. If the sauce gets too thick and salty, add water and heat it up.