In the bowl of a stand mixer, beat together egg whites, 1 teaspoon lemon juice, and 1/4 teaspoon salt on low speed until very frothy.
Increase speed to medium-high and gradually add sugar mixture, beating until stiff, glossy peaks form, 12 to 15 minutes, scraping down sides of bowl halfway through.
On a baking sheet lined with parchment paper, either pipe or mound the meringue into a roughly 8 or 9-inch circle. With the back of spoon, create a crater in the centre of the circle (this will hold the cream filling after the pavlova is baked).
Bake the meringue for 2.5 hours (it should be solid on the outside and appear dry). Turn the oven off and leave the pavlova in the oven overnight to cool.
When you are ready to assemble the pavlova, beat the cream and vanilla extract until you have stiff peaks.
Remove the pavlova from the parchment paper and place the pavlova on a serving tray. Spoon the whipped cream mixture into the centre of the pavlova.
Top with the reserved strawberry/rhubarb topping (if you want, you can spoon the juices over as well but remember that this will soften your pavlova quickly so try not to allow too much juice on top of the pavlova).
