How to Make It
Preheat oven to 250°F.
Prepare meringues: Beat egg whites and salt with an electric mixer fitted with whisk attachment on medium-high speed until foamy. Add cream of tartar; beat until soft peaks form.
Scrape vanilla bean seeds from pod into a small bowl; discard pod. Whisk together sugar, cornstarch, and half of the vanilla seeds in a second small bowl. Set aside remaining vanilla seeds. Gradually add sugar mixture to egg white mixture, beating on medium-high speed until stiff, shiny peaks form, about 5 minutes (do not overbeat).
Line 3 baking sheets with parchment paper, and spread meringue mixture into 3 (8-inch) circles on prepared baking sheets. Bake at 250°F 1 hour and 15 minutes. Turn oven off, and open oven door just slightly. Let meringues stand in oven 30 minutes.
Prepare filling: If using, combine lemon thyme leaves and 2 tablespoons sugar in a small bowl; rub mixture together using your fingertips. Place peach slices in one bowl and raspberries in a second bowl. Sprinkle sugar mixture evenly over peaches and raspberries; toss each to combine.
Whisk together yogurt, crème fraîche, remaining vanilla bean seeds, and remaining 2 tablespoons sugar in a small bowl until smooth.
Assemble the pavlova cake: Place 1 meringue round on a cake stand or platter. Spread with a thin layer of yogurt mixture, and top with about three-fourths of the peaches. Top with second meringue round. Spread with a thin layer of yogurt mixture and top with about three-fourths of the raspberries. Top with third meringue round. Spread with remaining yogurt mixture, and arrange remaining peaches and raspberries on top of pavlova cake. Garnish with lemon thyme sprigs. Serve immediately.