Preheat the oven to 120 °C / 250 °F. In a large bowl whisk the egg whites with the lemon juice using a hand mixer until they form soft peak, for about 2 - 3 minutes, then whisk in the vanilla seeds and sugar, 1 tablespoon at a time, until the meringue forms stiff peaks and becomes glossy. Add the corn starch, mix for another second.
Spread the meringue out to a 16 - 20cm ( 6 - 8 inch) circle (or make two small ones) on the baking sheet lined with parchment paper. Dust with icing sugar and place it in the oven. Bake for 3 hours then turn off the heat and let the Pavlova dry slowly in the oven for another 20 minutes. Once baked, transfer it to a wire rack.
Prepare the topping. Clean the strawberries and cut the large ones in half. Whip the whipping cream softly. In another bowl soften the mascarpone cheese using a spatula. Slowly start add the whipped cream. Spread the cream - mascarpone mixture evenly over the pavlova, put the strawberries on top and serve.