For the crumble
for the custard
Put the flour, sugar, almonds, ginger and salt into the bowl of an electric mixer or food processor.
Gently blend these dry ingredients together and then add the butter.
Mix as for starting to make pastry, but do not work the mixture too much; the look should be a touch lumpy rather than the usual finer-textured 'breadcrumb' look.
Preheat the oven to 180°C
Spread the rhubarb into a suitable baking dish and sprinkle with the sugar.
Pile the crumble mixture over the top, but do not pack down; give the dish a sharp rap on the worktop to settle the surface.
Briefly drag a fork over the surface to give texture.
Put on the middle shelf of the oven and bake for 30-40 minutes until the surface is golden and slightly blistered brown in parts.
To make the custard, heat the milk together with the vanilla pod in a heavy-bottomed saucepan.
Remove from the stove and whisk for a few seconds to release the vanilla seeds into the milk.
Briefly beat together the egg yolks and sugar and strain the hot milk over, whisking as you go.
Return to the saucepan and cook over a very low heat (with a heat-diffuser pad if possible) until limpid and lightly thickened. Some say it should coat the back of a wooden spoon, but I don't go along with this theory; it should be taken further than this, almost until there is the odd simmering blip on the surface. When you think it is ready, give a final vigorous whisk to amalgamate and pour into a warm jug
Note: if you are unlucky enough to split the custard, a blast in a liquidizer will usually rescue it.