The best dish I can make with a bunch of baby horse mackerel is "Aji no Nanban-zuke" : marinated baby horse mackerel.
"Nanban-zuke" means "marinade in Nanban style". "Nanban" is an old Chinese word and means southern barbarians. When Portuguese and other Europeans came to Japan for the first time as missionaries, sailors and merchants in the 16th century, they were called Nanban, because they came to Japan from the southern direction.
In Japan, the word Nanban has lost the original discriminatory meaning and it is nowadays used to describe once novel and interesting things which were brought by Nanban people, in particular in relation to food. Nanban-zuke is one of the novel cooking methods introduced into Japan some four hundred years ago.
There are several novelties as to Nanban-zuke. Onion and chili were alien to Japanese people at the time. Frying with oil and marinating fried meat or fish were both new cooking methods.
Let me start with cutting onion, carrot and green pepper into thin strips. Put about two thirds of the vegetables into a plastic container and keep the rest for the later use.
Put vegetables in a plastic container
Then, prepare fish. Take out innards from mackerels. If mackerels are fairly grown up, it is advisable to take off the hard scales of the lateral line.
Then remove them from the frying pan and put them on the vegetables. Pour the sauce while fishes are still hot and put the rest of vegetables on top of them. Then close the lid and keep the container in the refrigerator. You can eat Nanban-zuke after an hour or so. However, I prefer to marinate for several hours so that the taste becomes mature.
The most troublesome part of the cooking is the preparation of fish. When I have guests in Izu, I usually prepare 30 to 40 small fishes and it is really tiring. However, this time, fishes are those I angled myself and it was rather a fun to prepare them. Then, they tasted somehow better than usual.