Preheat the oven to 180°C/gas 4.
To make the crumble topping, put the oatmeal, oats and butter in a food processor and pulse briefly until the butter is worked into the oatmeal and the mixture resembles fine crumbs – don’t overdo it, or it will start to clump together too much. Tip into a bowl and stir in the demerara sugar.
Wash the rhubarb and cut it into 1–2cm pieces. Spread it in an ovenproof dish (about a 2 litre capacity). Squeeze the orange juice over it, or sprinkle on the apple juice, then evenly scatter over the caster sugar. This amount of sugar should be fine for new season, forced rhubarb – the slender-stemmed, bright pink stuff. If you’re using robust, outdoor-grown rhubarb later in the year, you might need a little more.
Spoon the oaty topping over the rhubarb in an even layer. Bake for about 45 minutes until golden brown on top and bubbling underneath. Serve hot, warm or cold with cream, yoghurt or custard.
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