Start with enough fillets(I scale and leave skin on,optional)that will fill a qt jar
2/3 full when cut into bite-size pieces. Can double/triple this recipe.

Soak fish pieces in a cold place for 24 hrs in brine made of 1 c pickling salt to 6 cups water. Drain, then cover with white vinegar for 24 hrs in cold place.Drain.
Pack jars with alternating layers of fish and onion slices. Cover with pickling sauce made of 1 c white sugar
1 c white vinegar
2 Tbsp mixed pickling spices-gently boiled/stirred for 5 min and
allowed to cool.
Keep in fridge and enjoy. I have kept them for up to 8 mos. with no problem.
Pretty much like pickled herring in wine sauce. Let sit a couple weeks before eating for pickling to penetrate.

I have added a few garlic cloves, peppercorns, dill sprigs, and a jalapeno or 2
split and seeded(I boiled them with the sauce to be safe).