Preheat your oven to 300 degrees F (150 degrees C). Line a baking tray with greaseproof paper. Put the egg whites into a bowl, making sure there are absolutely no little pieces of shell or yolk in them. Whisk with an electric hand whisk or freestanding whisk on a medium setting until they form firm peaks. You’ll know that it’s thick enough if you can hold the bowl upside down over your head!
With your mixer still running, gradually add the sugar and salt. Turn the whisk up to the highest setting for about 7 or 8 minutes, or until the meringue mixture is deliciously white and glossy. Dip your finger in the meringue and rub the mixture between your thumb and index finger. It should feel perfectly smooth. If it feels at all grainy, whisk for a little bit longer. Keep a close eye on the meringue, because if you whisk it for too long, it will collapse.
Spoon dollops of the meringue on to the prepared baking tray — you’ll get about 6 to 8 meringues from this amount — leaving a bit of space between them. Dab them with the bottom of your spoon so you get wispy bits coming up from the surface of the meringue. Pop the baking tray into the oven for around 1 hour, or until the meringues are crisp on the outside and chewy in the middle. Leave to cool.
Meanwhile, whip the Jersey cream with the vanilla seeds and 1 tablespoon of sugar until you have soft peaks. In a bowl, mash half the strawberries and half the raspberries with the rest of the sugar and the balsamic vinegar. Pop both bowls in the fridge until your meringues have cooled down.
You can serve Eton mess on a large platter on in individual glasses. To assemble, break up your meringues into a bowl — you can crush some of the bits into powder, leaving other bits chunky. Ripple the cream and mushed-up fruit together, then sprinkle in the rest of the fruit and fold again. Layer your crushed meringues and fruity cream onto your serving platter or in glasses, then sprinkle with the toasted almonds, if using. Serve right away.