First make the ice cream: put the milk into a saucepan, split the vanilla pod lengthways with a sharp knife and scrape the seeds into the milk with the point of the knife, then add the pods as well. Bring the milk to the boil and simmer very gently for a couple of minutes, stirring a couple of times, then remove from the heat.
Whisk the egg yolks and sugar together (using a hand whisk, not the electric type) then pour the milk on and whisk well. Return to the pan on a low heat and cook for about five minutes, stirring constantly with a whisk, but don’t let it boil. Remove from the heat, whisk in the cream and remove the vanilla pods.
Leave to cool. This can be speeded up by standing the mixture in some cold water chilled with a frozen ice pack. When it’s cool churn the mixture in an ice-cream machine. Some machines need putting in the freezer first so make sure you’ve done this in time. Meanwhile pre-heat the oven to 200C/Gas mark 6
To make the topping, rub the butter into the flour with your finger tips until the mixture has a breadcrumb-like consistency. Mix in the sugar, almonds and oats. Spread the crumble on to a baking tray and bake for 10-12 minutes until golden, then leave to cool.
Meanwhile put the rhubarb in a heavy-based saucepan with the sugar and cook on a low heat with a lid on for 6-7 minutes, stirring every so often until the rhubarb is just cooked but not disintegrated. Drain off the juice into a small saucepan and simmer until it’s thick and syrupy then mix with the rhubarb and leave to cool.
When the ice cream has almost set, stir in the rhubarb and the crumble mixture. You can add the mixture to the machine, or remove the ice cream, and pour it into a freezable container then stir in the crumble and leave it in the freezer to set for a couple hours before serving.