Rhubarb

Not optional. The stalk is mostly malic acid and it wins every argument without sugar.

Custard. Sour needs fat and sugar arriving together, not one after the other.

Heat from the root, acid from the stalk. Neither is sweet and it still reads as pudding.

The strawberry supplies the sugar and perfume rhubarb has none of.

Zest, not juice. Rhubarb has all the acid it needs and none of the perfume.

Fat blunts the astringency that makes your teeth feel chalky.

Rounds the sour edge without adding another flavour to argue with.

Both peak in the same six weeks. The muscat note is what rhubarb lacks.

Two drops. Rhubarb is loud enough to carry perfume that would drown anything else.

Frangipane under the fruit soaks the juice that would otherwise wreck the pastry.

Northern European logic: oily fish wants a sharp fruit, and gooseberry is not the only option.

Apple sauce with the sugar removed. Cuts fat harder and tastes less like a compromise.

Sourer than cherry, sharper than orange. The fat can take it.

Lactic tang against oxalic tang. Two different sours that do not cancel.

Stewed together the burn reads as a spice you cannot name, and the fruit stops being flat.

Resin plus acid reads almost like gin. Serve it with something cured.

Cracked over the roasted stalks. Pepper finds the green note nobody looks for.

Salt-fermented depth under a fruit this sour makes it read savoury, not sweet.