Line a large baking sheet with baking parchment. Using an 18cm round cake tin or plate as a guide, draw a circle on the parchment, then flip it over. Heat the oven to 130C/110C fan/gas ½.
Rub the reserved lemon half around the inside of a large bowl or stand mixer, then wipe with kitchen paper to remove any butter or oil. Tip in the egg whites and whisk with an electric whisk or the mixer on a low speed until the whites form small, foamy bubbles, about 1-2 mins. Keep going until the mix looks like shaving foam, then add the sugar, 1 tbsp at a time, until it’s a thick, glossy meringue. Rub some between your fingers – if you can feel any grains of sugar, keep whisking for a few minutes. Add the cornflour, lemon juice and vanilla, and whisk for another 30 seconds.
Spoon the meringue into the centre of the circle and spread it out to the edges with a palette knife. Swipe the knife around the edge, then smooth the top.
Bake for 15 mins, then turn the oven down to 100C/80C fan/gas ¼ and bake for 1 hr. Turn the oven off and leave the pavlova inside to cool for at least 2 hrs, or overnight.
When ready to serve, carefully invert the pavlova and peel away the parchment. Don’t worry if it cracks a little – it will be covered by the topping.
To make the topping, heat the double cream with the milk in a small pan until steaming. Remove from the heat. Whisk together the egg yolks, cornflour, vanilla extract and caster sugar in a bowl. Slowly pour this over the cream mixture, stirring continuously, then return the pan to a low-medium heat. Cook for 5 mins, stirring, until thickened – it should coat the back of a spoon and leave a channel when you run a finger through it. Transfer to a bowl and cover the surface with baking parchment to prevent a skin forming. Chill for at least 1 hr, until thick enough to spread.
Pit and halve the cherries (leaving a few whole, if you like). Toss with the icing sugar and a dash of cherry liqueur or more vanilla extract, then leave to macerate for at least 30 mins. Spoon the cherry jam or compote over the centre of the pavlova, and spread it out to the edges. Beat the chilled custard briefly, then spread it over the jam. Top with the macerated cherries and scatter over the toasted flaked almonds. Best served straightaway but leftovers will keep in the fridge for two days.