To make the pavlova:
Preheat oven to 225°F and line a sheet pan with parchment paper.
In the bowl of an electric mixture, beat the egg whites on medium-high speed.
Combine the sugar, cornstarch, cream of tartar and salt in a small bowl. When the egg whites form soft peaks, slowly add the sugar mixture. Continue to beat the egg whites until glossy and stiff peaks form (This may take longer than you think, "stiff peaks" means the meringue stands up straight off the whisk without flopping over). Gently fold in the vanilla extract.
Spoon the meringue onto the parchment paper (I used a medium 3-tablespoon cookie scoop). Use the back of a spoon to gently make a little well into the center of each meringue.
Bake for 1 hour 15 minutes without opening the oven door. Pavlovas will loose their gloss and look dry on the outside. Turn off the oven and let the pavlovas cool like that for 30 minutes. Then, remove and let cool completely.
When ready to serve, whip the whipping cream, adding the sugar and flavoring during the last 30 seconds or so of whipping. Spoon over the baked meringue and top with berries.
To make the berry compote:
While the pavlova is baking and cooling, make the berry compote. Add fruit, sugar, lemon zest and lemon juice to a small saucepan. Bring to a boil, then reduce heat and simmer for 8-10 minutes until berries start to burst and soften.
Combine cornstarch and water to make a slurry. Add to the saucepan and stir to combine. Continue to cook for 1-2 minutes until thickened.
Remove from heat and let cool completely. If making ahead, store in the fridge until ready to serve.
To make the whipped cream:
To assemble mixed berry pavlovas:
Calories: 163kcal, Carbohydrates: 23g, Protein: 2g, Fat: 7g, Saturated Fat: 5g, Polyunsaturated Fat: 0.4g, Monounsaturated Fat: 2g, Cholesterol: 22mg, Sodium: 27mg, Potassium: 110mg, Fiber: 1g, Sugar: 22g, Vitamin A: 296IU, Vitamin C: 19mg, Calcium: 20mg, Iron: 0.2mg