Grapefruit Treacle Tart

I adore treacle tart. At lunch before my wedding, I had 24 oysters followed by a bowl of treacle tart and custard. Bliss. But I do recognize that most treacle tarts are too sweet, so I devised this variation in which grapefruit cuts through the sweetness of the syrup. Serve it with ice cream, cream, or good old-fashioned custard, homemade or shop-bought.