My starting point was Linda Ziedrich’s Joy of Pickling recipe. I adapted it slightly.
3 cups distilled white vinegar
1 cup water
1 cup sugar
3 T. allspice berries, cracked (use a mortar and pestle, or put them in a bag and use a hammer)
2 T. whole peppercorns
4 bay leaves
1 ½ lbs. filleted salted herring, cut in bite-size pieces (see NOTE, below)
2 large onions, sliced
In a small saucepan, bring the vinegar, water, sugar, allspice, peppercorns, and bay leaves to a boil. Turn down the heat and simmer for five minutes, and then let cool to approximately room temperature (that’s so it doesn’t cook the fish when you pour it in).
In a jar big enough to hold everything, pack the onions and herring in alternating layers. Pour enough brine into the jar so that everything’s covered, and then knock the jar on the counter to free any air bubbles. Pour in more brine if you need to. As you pour, most of the allspice, peppercorns, and bay leaves will tend to stay behind in the pan. That’s fine – you want some of it in the jar (mostly for looks), but probably not all of it.
Refrigerate at least 48 hours. It should last in the fridge for a month or so.
NOTE: I used fresh herring that had been stored in brine, and I soaked them overnight, in two changes of water. If you buy salt herring, de-salt it according to the instructions before you use it.