Put the flour in a large bowl, add the ground almonds, sugar and a pinch of salt, and mix to combine] Coarsely grate or finely dice the butter, then rub it into the dry ingredients with your fingertips, until the mix is fairly well combined but still lumpy – you shouldn’t be able to see butter, but nor should the mixture be homogenous.
The fruit will be hidden underneath the topping, so crumble is also a great use for less lovely thicker or greener stems of rhubarb, underripe stone fruit, gluts of apples, etc.
Pour the demerara sugar evenly over the rhubarb and toss well to coat (you can use white or soft brown sugar, if you prefer, with the caveat that the darker the sugar, the darker the result.
Nigella reckons rhubarb is “the best crumble in the world”, but the recipe can, of course, be adapted to other fruit, according to both taste and season (or, indeed, to frozen fruit at any time of year); for other options see step 8.
Meanwhile, roughly chop the extra nuts, if using, then, when the half-hour is up, scatter these on top of the crumble and bake for 10 minutes more, until the topping is crisp and golden and the fruit below bubbling.
