Ooooh, so that's it... I based my whole translation on the supposition that they were grilling seaweed to it. It looks like they were extracting salt from it instead. Man, I feel so stupid now. I swear this chapter will be the end of me...
Thank you so much! You have been immensely helpful! I didn't know real life Narihira did something like this. Well, you learn something new everyday!
I'll read it over again and upload a fixed version either later today or tomorrow (probably tomorrow because I'm a little busy).
They don't extract salt from it. The seaweed is served together with the grilled fish. It needs to be carried in saltwater to preserve it (seaweed will rot if exposed directly to air). As weird as it sounds, since the salted fish is too salty, you're supposed to wash off the salt with brine, which is the other use for the saltwater they brought. Apparently washing salted fish with freshwater damages the skin.
The actual subversive message of shioyaki has to do with the way the salt trade was governed in Heian Japan. In the Heian era, one of the innovations adopted from China was the creation of salt commissioners, whose job was to control the production of salt and monopolize the trade, thereby controlling the price of salt and giving the central government a steady income. Well, unlike China Japan is an island country, and just about every seaside community produced their own salt, so the salt commissioners had a really rough time doing their job. In fact, a 2015 assessment of the economy of Japan during the Edo period concluded that salt formed a massive chunk of the unrecorded economic activity of Japan and that salt smuggling into China was one of the most important trading relationships of the era. Presumably, the situation in the Heian era was wilder, since Tokugawa control of Japan was far more robust than that of the Heian court.
Basically, a Heian noble inviting peasants to grill food with their own salt was publicly flaunting his political immunity, since he's inviting people who are clearly breaking the law to make the delicacy he's eating.
isn't it dangerous for him denouncing a bunch of nobles?
A bunch of OUT OF POWER nobles, so no. In fact, at this time Sugawara no Koreyoshi was higher-ranked than any of them, both in office and noble rank. The "wara" families were all the same rank and Koreyoshi was head of the Sugawara. The historical Koreyoshi was actually far more powerful than depicted in this manga, as his power was only surpassed by the Fujiwara.