Preheat oven to 150 degrees Celsius/300 degrees Fahrenheit.
Cut the rhubarb into 1-inch pieces and place in an oven-proof dish. Sprinkle the sugar over the top, and grate and squeeze over the orange zest/juice.
Bake rhubarb in the oven for 20 minutes or until it is tender and a knife goes in easily. Remove and allow to cool.
Pistachio meringue
Turn the oven up to 160 degrees Celsius / 320 degrees Fahrenheit and prepare four baking trays lined with parchment paper.
Whip egg whites using an electric hand whisk or food processor until they go opaque white and reach stiff peaks. Gradually add the sugar whilst continuing to whisk until it is all incorporated and the meringue mixture is glossy and white.
Fold in the ground pistachio nuts with a rubber spatula to avoid knocking out too much air.
Spread round, flat disks of pistachio meringue onto each baking sheet.
Bake the pistachio pavlova layers in the oven for 20 minutes, then turn the oven off and allow it to cool completely before opening it to allow the meringue to dry out.
Assembling rhubarb and pistachio pavlova
Whip heavy cream until it is thick and gloopy enough to stay still but not stiff, and chop pistachio nuts into small pieces.
Place a pistachio meringue pavlova layer on a serving platter and spread whipped cream over the top right to the edges. Lay another meringue disk on top, and repeat.
Use a slotted spoon to place the stewed rhubarb on top of the cream and pistachio pavlova stack and sprinkle with pistachio nuts. Just before serving, pour over the rhubarb syrup/cooking liquid as an extra sauce that tastes delicious with the other flavors.
Video
Notes
If you cannot find ground pistachio nuts in stores or online (try Amazon), then you can make them yourself. Roast the same weight of shelled unsalted pistachio nuts on a baking tray for 5-10 minutes and then blend in a food processor. You can bake the meringue disks in two batches if you can't fit all the trays in the oven at once. I recommend baking the first batch for 20 minutes, turning the oven off, and waiting another 40 minutes before taking them out to bake the remaining two disks. I always recommend weighing ingredients in grams rather than using the cup system, which is not precise enough for baking.