Lay herring fillets face-down (skin side up) on a generous layer of a 3:1 mixture of kosher salt & granulated sugar (a.k.a., “salazon”). Cover in plastic and refrigerate like this overnight.
The following day, remove fillets from the salazon, rinse & pat dry. Submerge the salted fillets in apple cider vinegar. Weigh fillets with a plate to keep them submerged, cover, and refrigerate overnight.
The following day, remove fillets from the vinegar, pat dry, and place in sanitized pan or jar. Cover in layers with thin sliced yellow onion & extra-virgin olive oil.
These will keep in the refrigerator for a few months if the oil cap is kept over the fillets.
Use in any application which calls for boquerones.