Deep dive
Why some sardines are aged like wine
Certain houses date their tins. Turned every few months, the fish softens and the oil sweetens.
A sealed tin is a slow kitchen. Fat from the fish migrates into the olive oil, the oil migrates back, and the texture keeps changing for years.
Cellars that age conservas turn the tins every few months so the fish stays evenly bathed. After two years the fillets are noticeably softer and the oil noticeably rounder.
You do not need a cellar. A dark cupboard and patience do the same job.