Editor’s note: The theme of Gohan Lab is to help people make simple, tasty “gohan” (meals).

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Here's a great way to enjoy seasonal fish simmered in sweet soy sauce.

Though we're going to use flounder fillets in this “nitsuke” recipe, the basics of it apply to all fish, be they plain fish with white flesh or fatty blueback fish, fillets or fish served from head to tail.

Ryohei Hayashi, who supervises the cooking aspect of this week's recipe, has determined an “equation” for simmering fish. Why not try it on the fish of the day recommended at your local supermarket?

After immersing the fish in hot water to remove sources of odor such as blood and stickiness on the skin, a process called “shimofuri,” we'll weigh it to determine the right amount of liquid mixture to prepare for the sauce to simmer it.

The liquid should weigh the same as the fish. The soy sauce to use should weigh 10 percent of the fish, and the sugar required should weigh half of the soy sauce.

Our recipe uses a half-water, half-sake ratio to make the liquid mixture used at restaurants and tastes lightly sweet overall. You can adjust the flavor to suit your taste the next time you make the dish.

Keep the heat high while simmering and cook the fish by enveloping it in the bubbling sauce. Remove the fish while fluffy, reduce the sauce further and pour it over the fish. The flavor won't have seeped into the fish, but should be enjoyed by dipping the fish into the sauce.

Our recipe's arranged version features diced tuna. The dish turns out quite differently if you increase the simmering time and amount of seasonings.

SPECIAL POT FOR SIMMERING

A country's cuisine and cooking utensils are closely connected.

In Japanese cuisine, “niru” is a wide-ranging cooking method meaning to boil or simmer.

In “nitsuke,” the simmering sauce is brought to a boil and the ingredients are cooked in a short time, while in “teri-ni,” the sauce is cooked away at the end. The dishes turn out differently depending on the heat's strength and the amount of sauce.

A “yukihira” is a pot excellent for boiling or simmering. Made of material with high thermal conductivity such as copper and aluminum, the pot has small dents on the surface made by the “uchidashi” process that increases the surface area exposed to heat.

The pot's drop lid with appropriate weight keeps the ingredients from falling apart while being simmered.

BASIC COOKING METHOD

(Supervised by Ryohei Hayashi in the cooking aspect and Hiroya Kawasaki in the cookery science aspect)

* Ingredients (Serves two)

2 fish (flounder) fillets (200 grams), 100 ml water, 100 ml sake, 20 grams (little more than 1 Tbsp) soy sauce, 10 grams (little more than 1 Tbsp) sugar, some yuzu zest

About 175 kcal and 1.8 grams salt per portion

1. Make a cross-shaped incision on thick part of the side of the fish that will face up when served. Flounder looks best when its white stomach side faces up. Bring water to a boil in a pot and have ice water ready in a bowl. Turn off heat, place a fillet on skimmer and immerse in hot water (PHOTO A). As the surface will turn white right away, pull out and pour in ice water. Repeat process for second fillet and remove.

2. After pouring out hot water and washing pot, add water, sake, soy sauce and sugar and mix. Lay fillets side by side (PHOTO B), cover with drop lid and turn stove to high heat.

3. When pot comes to a boil, keep the heat high and set timer for 4 minutes. Keep heat so the bubbling sauce reaches above the drop lid (PHOTO C). When timer rings, turn off heat and serve fish on a plate.

4. Place pot with simmering sauce on medium heat. Decide how far you wish to reduce the sauce, so it is runny or thick. Pour sauce on fish and top with fine strips of yuzu zest.

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Ryohei Hayashi is the owner-chef of Tenoshima, a Japanese restaurant in Tokyo’s Aoyama.

Hiroya Kawasaki studies the science of tastiness and cooking methods in collaboration with top chefs at Ajinomoto’s Institute of Food Sciences and Technologies.

ARRANGED VERSION

Tuna in “shigure-ni” and “tsukuda-ni” styles are prepared meals made of red tuna or skipjack tuna. The lighter “shigure-ni” is made by cutting 200 grams of tuna into dices 2 to 3 cm on the side and simmering with the same seasonings as the above nitsuke until no fluid remains.

You can also add a piece of ginger half the size of thumb cut into fine strips. The dish keeps in the fridge for 4 to 5 days. To make “tsukuda-ni,” simmer the same amount of tuna, water and sake with double the amount of soy sauce and sugar. This version is good for about 10 days in the fridge.

COOKERY SCIENCE

When the pot is covered with a drop lid and cooked over high heat when making a nitsuke, the convection flow of the simmering sauce becomes active and the fish is heated from above as well.

If you pour the identical amount of simmering sauce into a frying pan with the same diameter as the pot, the convection flow weakens due to its different shape. 

Cooking the fish in a pan reduces how much it gets heated from above, prolongs the cooking time and makes the fish become firmer.

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From The Asahi Shimbun’s Gohan Lab column