Whisk together the grapefruit zest, grapefruit juice, sugar, cornflour, salt and egg yolks in a pan. Bring to the boil over a medium heat, whisking constantly. As soon as the mixture thickens, remove the pan from the heat, add the butter and continue whisking for approx. 2 mins. Pass the curd through a sieve and into a bowl, leave to cool, then cover and chill for approx. 30 mins.
Beat the egg whites with the salt until stiff. Add half the sugar and continue beating until the egg whites become very stiff and glossy. Add the remainder of the sugar and beat briefly. Transfer the mixture to a piping bag with a serrated nozzle (approx. 9 mm in diameter), pipe little blobs onto a baking tray lined with baking paper.
Slide the tray of mini meringues into the lower half of an oven preheated to 120°C, reduce the heat to 100 °C and bake for approx. 30 mins. Then turn off the oven and leave the meringues to cool with the oven door open.
Whip the cream with the cinnamon until stiff, transfer to a piping bag with a serrated nozzle (approx. 9 mm in diameter). Spread the quark over the base of the tartlets, top with the curd and smooth down. Add the meringues, pipe the cream on top. Sprinkle with the pistachios.