Prepare meringue cookies at least 3 hours in advance of beginning (I like to prepare mine a day in advance). Set aside.
To prepare berry layer, combine 2 cups (about 300g) of quartered strawberries (please note this is not all of your berries, only 2 cups. Reserve the remaining berries, you'll need them shortly), sugar, cornstarch, and lemon juice in a medium-sized saucepan.
Stir until ingredients are well-combined. Turn stovetop heat to medium/medium-low and cook, stirring constantly, until berries release their juices.
Continue to cook (still stirring constantly) until mixture begins to boil and slightly thickens.
Remove from heat and stir in your remaining berry slices. Allow mixture to cool completely before using in Eton mess.
Prepare whipped cream by combining cold heavy cream, powdered sugar, and vanilla extract in a large bowl.
Use an electric mixer to beat until thick, fluffy, and stiff peaks have been reached. If not using immediately, store in an airtight container in the refrigerator until ready to serve.
Note: do not assemble Eton mess until you are ready to serve. Place your meringue cookies in a large ziploc bag and use your hands or a rolling pin to crush them into bite-sized or smaller pieces.
Add approximately ⅔ of the meringue pieces (no need to measure, just an approximation is fine) into the whipped cream and use a spatula to fold together.
Dollop a spoonful of the whipped cream/meringue mixture into the bottom of a (preferably chilled) trifle glass.
Sprinkle a spoonful (I use about a Tablespoon) of the remaining crushed meringue over the whipped cream mixture.
Top with a generous spoonful of the berry mixture. Repeat until trifle glass is full. Serve immediately.