1. Preheat oven to 350ºF. In the bowl of a standing mixer fitted with the whisk attachment, beat the egg whites, water, vanilla and cream of tartar on slow speed for 1 minute. Raise the speed to medium and slowly add the sugar, beating until stiff peaks form.
2. Using a wire mesh sieve, sift the flour and salt into a bowl. Spoon out a quarter of the flour mixture and, using the sieve, sift over the egg whites. Using a spatula, gently fold the flour and egg whites together. Repeat until all of the flour has been used.
3. Carefully transfer the mixture to an ungreased angel food cake pan. Bake for 35 minutes, or until golden brown and top springs back when lightly touched.
4. Immediately invert on a cooling rack (or over the neck of a wine bottle) for 1 hour. Run a knife around the edge of the cake, before removing from the pan. Dust with confectioners sugar, if desired.
5. To make the topping, combine the fruit, sugar and thyme, if using, in a large bowl. Whip the cream and yogurt together until it forms stiff peaks. Garnish the cake with the yogurt cream and fruit. Drizzle over any juices and garnish with thyme sprigs, if desired.
Per serving: 223 calories, 4g fat, 3g saturated fat, 15mg cholesterol, 106mg sodium, 42g carbohydrate, 1g fiber, 28g sugar, 5g protein