I've been thinking about cassis all day and wondering quite what to match it with until I was reminded that Hercule Poirot was fond of it - which makes perfect sense for the character - I can see him now liqueur glass in hand, appreciating the finer points of a good cassis.
It does need to be a good cassis though, my current preference is for White Heron British Cassis, which is twice the price of the French one we sell, but much better. The reason it needs to be good, and that it's worth spending more on, is that as it's bottled at only 15% it won't keep its freshness especially long once it's open*. You need a bottle you want to drink.
just going over the hill, if it's got a definite fault get rid of it).
There are plenty of other Cocktails that call for Cassis, and it's a handy kitchen ingredient too, adding a bit of booze blackcurrenty glamour to all sorts of things, and a great potential match with something like a chocolate tart, or very dark and rich chocolate cake or torte (beware to many other flavours though). It can be a port alternative with the cheeseboard as well, or just good on its own at the end of a meal, or the end of the evening - it's versatile stuff.
I think Poirot might have looked askance at an English version, but he would be far to particular to accept a lesser quality Cassis, and once he tried White Heron I know he'd approve. What Agatha Christie's personal opinion of Cassis was is a mystery I haven't yet looked into.
* It's the case with a lot of liqueurs, vermouths, and other fortified wines that they really don't keep as well as people think. Contact with oxygen destroys the freshness of the flavours, which is really noticeable with something fruity where you definitely want that freshness and vibrancy. To get the best out of them refrigerate after opening and aim to use within a month. Unless it smells really bad, or has developed mould, it won't do you any harm to drink it after this point (cream based liqueurs are another matter) but it won't taste as good.