To make the meringue:

1. Preheat the oven to 250F and line a baking sheet with parchment paper.
2. Whisk the room temperature egg whites to stiff peaks on high speed in a clean bowl, either with a hand mixer with whisk attachment or stand mixer with whisk attachment.
3. Add in sugar in 3 portions (⅓ cup each addition), re-whisking on high speed to stiff peaks after each addition. You should end up with a glossy white, egg white mixture with stiff peaks.
4. Sift the cornstarch on top of the mixture and add the vanilla extract and lemon juice. Gently fold in the ingredients with a spatula to combine.
5. Spread the mixture onto the parchment paper lined baking sheet, about ½ inch thickness.
6. Bake on the lower rack of the preheated oven for 1 hour to 1 hour 15 minutes until meringue is completely hard on the outside and a corner of it will lift off the paper easily. Start checking around the 1 hour mark of cooking time.
7. Once baked, let cool fully on the baking sheet.

To assemble the Eton Mess:

1. Once the meringue has cooled, make the whipped cream by using a stand mixer or hand mixer fitted with the whisk attachment. Add 1 ½ cups cold whipping cream and 2 tbsp sugar. Beat on medium-high speed until fully whipped in soft peaks and spreadable. Set aside.
2. Make the strawberry and raspberry mixture by adding strawberries, raspberries, lemon juice and sugar to a bowl. Use a fork to slightly crush the raspberries and strawberries and then mix everything together until the sugar is blended in. Set aside.
3. Crush the meringue into pieces using your hands.
4. To assemble the eton mess, in a glass or on a plate, start with a layer of whipped cream followed by a heaping spoonful of the berries and top with crushed up meringue pieces. Repeat the layers once more, dividing everything evenly over 4-6 cups or plates, garnish with mint if desired and then enjoy!