Rinse, stalk and stone the cherries, then put them in a pan. Add 5 Tbsps of water and the vanilla sugar. Bring to the boil, then gently simmer for 5 minutes until the sugar dissolves.
Mix the cornflour with a spoonful of the cooking juice until you get a paste, then stir it into the cherries. When the syrup starts to thicken (but not too thick), turn off the heat, add the lemon juice and transfer the cherries and the sauce to an oven-proof dish.
In a food processor, blitz the oats until you get a coarse mixture. Add the butter and whiz until you get a mixture resembling breadcrumbs. Mix in the sugar and ground almonds. Scatter the mixture over the cherries, covering well. Sprinkle the almond flakes on top.
Bake for 25-30 minutes, until the crumble is golden and the cherries are bubbling around the edge.
Serve warm with double cream, custard, crème fraîche, yoghurt or vanilla ice-cream.