Pre-heat the oven to gas mark 5, 375°F (190Â°C). You will also need a shallow, 11 x 7 inch (28 x 18 cm) baking tin and a 3Â½ pint (2 litre) polythene freezer box, 8 x 8 inches x 2Â½ inches deep (20 x 20 cm x 6 cm deep).
First of all, make the crumble by combining all the ingredients together in a bowl and using your hands to rub the butter into the flour, so that the mixture comes together to form small, pea-sized balls of dough.
Now sprinkle this evenly into the baking tin and leave on one side. Now cut the rhubarb into Â½ inch (1 cm) lengths and place them in a large, shallow baking dish along with the sugar and lemon juice.
Place the dish on a lower shelf in the pre-heated oven and put the tin containing the crumble mixture on the shelf above.
The crumble needs to be baked for 10 minutes, then removed from the oven and left to cool. The rhubarb may need a further 15-20 minutes' cooking before it is completely tender: when it's cooked, take it out and leave it to cool a little before pouring it into a food processor or blender.
Process until you have a smooth pure, then pour it into a measuring jug, cover and transfer to the fridge to chill.
Before making the ice cream, use your hands to break up the cooled crumble and restore it to small, pea-sized pieces (if they're too big, the pieces are unwieldy to eat in the ice cream; if they're too small, they disappear).
Next, stir the cream into the rhubarb pure, pour into an ice-cream maker and churn until the mixture has the consistency of softly whipped cream.
Quickly spoon it into the polythene freezer box and stir in the crumble pieces. Put the lid on, then freeze for a minimum of 2 hours, or until the ice cream is firm enough to serve.
If frozen solid, the ice cream will need to be transferred to the main body of the fridge for about 25 minutes before serving to allow it to become soft enough to scoop.
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