Ideally we would never eat fish with a sauce.
Fish should be served jumping out of the water fresh. That's why there are large aquariums in good Chinese restaurants. That's the way it was in the old country, where my wife tells me fish was brought home from the store in buckets and left to swim until it was cooked
There is the family story of the carp that was brought home for dinner and left to swim in the bathtub. It was there for a few days, long enough for it to become a pet. This was not a good thing, as Martha Stewart might have said. We like to eat fish. We don't like to eat pets.
But this is not a story about keeping fish fresh. It is about creating the best sauce to serve with fish when it is not as fresh as we would like. And that's most of the time since Montreal is not a seaport.
A sauce should support the natural flavour of good fish. For some it may be a squeezing of lemon or maybe a combination of olive oil and lemon juice mixed with fresh oregano. Sometimes the sauce is thicker with an egg and oil base. These sauces have a stronger flavour that compliment a white fish such as flounder or snapper. They go great with almost any kind of fried fish. They are also delicious with cold cooked fish the next day.
My favourite fish sauce is the disreputable tartar. This sauce is almost never made fresh in restaurants. It is doled out in sealed packets and tastes either bland or unforgivably slightly rancid. It is not worth the plastic it is packaged in.
Tartar sauce is easy to make at home. It is a cold sauce based on mayonnaise. You can make your own but for tartar sauce I prefer to use a blander commercial brand, such as Hellmann's. I know the sharpness of the sauce will depend upon what I add to it.
Traditionally hard boiled egg yolks and chives are added to the mayonnaise. Other ingredients can include finely chopped pickles, crushed capers and minced shallots. The key is to balance the salty and sour flavours with the unctuousness of the mayonnaise. This is a strong and richly flavoured a sauce. Only a little is needed. The fish then becomes the base to transmit these flavours subtly.
You can experiment with the flavours you like but here is an easy version to start:
To one cup of mayonnaise add a quarter cup of finely chopped sweet red or white onion, a tablespoon of fresh chopped chives, a teaspoon of sweet relish or finely chopped dill pickle, a teaspoon of Dijon-style mustard, a couple of squeezes of lemon juice, a dash of tabasco, and salt and pepper to taste. Let this sit refrigerated for a few hours before serving.