In my grandmother Eva's kitchen in Kent, there was a week every July when the whole house smelled of hot sugar and sharp fruit. Gretel would arrive with a basket of red currants from the garden and the two of them would stand at the stove, stirring and tasting and arguing about when the jam was ready. Gretel always said you know it's set when a drop on a cold plate wrinkles under your fingertip. Eva said you know it's set when Gretel finally stops talking about it. They made this jam every summer for forty years.

Ribisel is the Austrian word for red currants. Not Johannisbeeren, which is what the Germans call them. Ribisel. If you walk through a Viennese Konditorei, this jam is everywhere, even if you don't see it. It's inside the Linzer Torte, sandwiched between Linzer Augen, spooned into Buchteln, stirred through Palatschinken. It's the invisible thread that runs through Austrian baking because its tartness does something no other preserve can: it cuts through butter and sugar and nuts and says, here, taste this, wake up.

The recipe is almost absurdly simple. Red currants. Sugar. Heat. Time. That's it. No added pectin, because red currants are loaded with their own. No lemon juice, because they bring more acidity than most fruits dare. You cook them down, strain out the seeds if you want a smooth jam (and for Linzer Torte, you want a smooth jam), and put it in clean jars. The colour is extraordinary. A deep, translucent ruby that catches the light like stained glass. Gretel used to hold the jars up to the window and admire them before she put them away. I still do the same thing.