Step 1Make the meringue base: Heat the oven to 425 degrees. Line a 15-inch by 10-inch jelly roll pan with parchment paper, so the paper rises 3⁄4-inch over the sides of the pan.
Step 2Place the egg whites in the bowl of an electric mixer fitted with the whisk attachment and beat on medium-high speed until soft peaks form, about 1 minute. Gradually add the sugar, 1 tablespoon at a time while beating, until the mixture turns into a thick and glossy meringue, about 5 minutes. Reduce the speed to low. Combine the vanilla, vinegar and cornstarch to form a slurry, and add to the meringue. Increase the speed to medium and beat until fully combined, about 1 minute.
Step 3Spoon the meringue into the lined pan and use a spatula to spread it out evenly. Place in the oven and immediately reduce the temperature to 400 degrees; the contrast in temperature helps create the crisp outside along with the gooey marshmallow-like inside. Bake until the meringue is pale beige in color and crusty on top, about 35 minutes. Remove from the oven and set aside in the pan until completely cool. The meringue will have puffed up in the oven but will deflate slightly when cooled. The meringue base (unfilled) can be prepared up to a day ahead. Leave it in the pan and drape with a kitchen towel until needed. You are then ready to fill it with the fruit and cream up to 4 hours before (but ideally as close as possible to) serving.
Step 1Make the filling: Using an electric mixer with the whisk attachment, beat the cream on medium-high speed until very soft peaks form, about 1 minute. Add the vanilla extract and powdered sugar and whisk to incorporate.
Step 2Place a kitchen towel flat on top of the meringue (or use the one that is already there, if you’ve made this the day before) and quickly but carefully invert it onto the work surface, so that the crisp top of the meringue is now facing down and sitting on top of the kitchen towel. Lift the pan off and carefully peel away the parchment paper. Spread 2/3 of the whipped cream evenly over the meringue. Cover generously with most of the sliced peaches, blackberries and almonds (save the rest for decorating the finished pavlova log).
Step 3Starting with the longest side closest to you, and using the kitchen towel to assist, roll the meringue up and over, so that the edges come together to form a log. Gently pull away the kitchen towel as you roll, then slide the meringue onto a long tray or platter, seam side facing down. Don’t worry if the meringue loses its shape a bit or some of the fruit spills out; just hold your nerve and use your hands to pat it back into the shape of the log.
Step 4Pipe or spoon the remaining whipped cream down the length of the log. Top with the remaining fruit and almonds, dust with powdered sugar and serve. The pavlova should be eaten on the day it is assembled and served, although leftovers can be stored in the fridge and eaten cold.