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1
Arrange the herring fillets flesh side-up in a single snug layer in a small baking dish. Season the flesh side with 1 tablespoon of the salt. Cover the dish and refrigerate for 24 hours.
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2
In a small nonreactive saucepan over low heat, combine the vinegar, brown sugar, garlic, peppercorns, mustard seeds, bay leaves, and remaining 1 tablespoon salt. Bring to a simmer and cook, stirring occasionally, to steep the aromatics and dissolve the sugar to create a brine, about 15 minutes. Strain the mixture through a fine mesh sieve into a nonreactive container. Add the onion to the brine, cover, and refrigerate until cold, about 2 hours.
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3
If you have a butane kitchen torch, gently run the flame over the skin of the fillets to tenderize. If you don't have one, the herring will still be good, but just a bit less tender on the skin side. Add the herring to the cold brine, cover, and refrigerate for 2 days.
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4
Pour off the brine from the herring and onion and discard the bay leaves. Separate the onions and the herring, then cover and refrigerate the herring. Transfer the onion to a large nonreactive bowl, add the sour cream, and stir gently to mix well. Cover and refrigerate for at least 1 hour before serving. The pickled herring and the onion mixture can be stored in the refrigerator for up to 1 week.
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5
Spoon pickled onion onto sliced rye bread and top with herring fillets. Garnish with dill and pepper. Serve immediately.
