1. Preheat the oven to 180°C, gas mark 4. Place the fruits in a saucepan with the sugar and the vanilla pod, add 2 tbsp water and gently heat with a lid on until the sugar has dissolved and the fruits start to break down. Tip into a bowl and chill until needed.
2. Whip the egg whites in a stand mixer or with an electric whisk with the cream of tartar and salt until foamy, then gradually add the caster sugar, whipping until the whites hold a medium peak when the beaters are lifted. Stir in the vanilla seeds, lemon zest and juice.
3. Add the sifted flour to the whipped whites in 2 additions and continue to whisk until incorporated. Scrape the batter into a 25cm ungreased angel food cake tin or 23-25cm non-stick ring mould, spread the top even with a spoon and bake the cake for 30-35 minutes until it springs back when gently pressed (try not to open the oven before 25 minutes).
4. Turn the cake pan upside down on a rack to cool the cake, and do not remove the cake from the pan until completely cooled. To remove, run a palette knife around the outside of the cake and around the inner circle, invert the tin and tap it on the surface until the cake drops out. Dust with the icing sugar and serve with the poached berries.