While I have a few Polish cookbooks, I'll try not to pull recipes out (besides, I don't own the copyrights and I hate citing works.) So, these recipes come from some housewives who learned them from their mothers (or mother-inlaws.) They miss some exact measurements, but that's half the fun; you can experiment with what you want and decide what you think is good.
Here are a few recipes for herring (sledz, pronounced like "shledge"):
The first one is an incredibly simple sledz in oil, which I find is rather popular (especially for Christmas Eve Supper.)
2-4 medium fillets of sledz (herring.) (Many Poles buy this from fish markets. The fillets are salted and technically edible as is.)
2-3 medium onions
peppercorns and capers are also optional.
Soak fillets in water for two hours and rinse (this is if you buy the sledz in the traditional way: salted.) Cut the fish into small pieces and pack into a sizable jar. Dice onions and pack on top (add other ingredients if you wish.) Fill jar up with oil and let sit for several days (it's good up to two weeks. I'd keep it in the fridge.) And that's it!
Pretty simple, right?
The next one actually isn't a Polish recipe, it's a Russian one. It's called "Herring in a Fur Coat" and has enough cholesterol to kill a pig. I think it's also a traditional New Years food there.
1-2 medium fillets of salted herring
3 medium onions
3 medium potatoes
4 small beets
2 large carrots
a large jar of mayonnaise
Get a good-size bowl (large enough to fit everything.) Peel and boil potatoes in salted water until cooked through (about fifteen minutes.) Boil the eggs until they are hard-boiled; boil beets until cooked through (but firm; maybe a half-hour.) Boil carrots until soft (ten minutes.) Peel eggs, potatoes, carrots, onions, and beets. Dice and fry the onions over medium heat until translucent (ten minutes.)
Get out your grater. Grate herring (if it's too hard, you can just mince it up as well) and layer the bottom of the bowl with it. Cover the herring with the onion and grate the potatoes over the onions for the third layer. Grate the egg whites over the potatoes; grate the carrots over the egg whites to form the fifth layer; grate the beets over the carrots to form the sixth. Add a good layer of mayonnaise atop the beets and top with the crumbled egg yolks. Cover and let it sit in the fridge for a few hours and let the mayo really seep down and sink in. Serve chilled.
You can mix up the layers and have multiple layers of herring or beets and whatnot. It ends up all mixed up anyway on the plate, but it's pretty good.