Heat 4 cups of water enough to dissolve salt. Let this brine cool to room temperature. When it does, submerge the herring fillets in the brine and refrigerate overnight, or up to 24 hours. Meanwhile, bring the sugar, vinegar, the remaining cup of water and all the spices to a boil. Simmer 5 minutes, then turn off the heat and let this steep until cool.
When the herring have brined, layer them in a glass jar with the sliced lemon and red onion. Divide the spices between your containers if you are using more than one. Pour over the cooled pickling liquid and seal the jars. Wait at least a day before eating. Store in the fridge for up to 1 month.