First of all make the crumble by combining all the ingredients together in a bow and using your hands to rub the butter into the flour s that the mixture comes together to form small, pea-sized balls of dough.
Sprinkle this evenly into an 11 X 7 inch baking tin and set aside.
Cut the rhubarb into 1/2 inch 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 an oven, preheated to 375*F, Place the tin containing the crumble mixture on the shelf above it.
The crumble needs to be baked for 10 minutes, then remove it from the oven and cool.
The rhubarb may need a further 15 to 20 minutes cooking before it is completely tender: when it is cooked, take it outand puree it in a food processor or blender til smooth.
Transfer to the fridge and chill.
Before making the ice cream, use your hands to break up the cooled crumble and restore it to small peas sized pieces.
(If they are too big the pieces are too unwieldy to eat, if too small,they tend to disappear.) Next, stir the cream into the rhubatb puree, pour it into an ice-cream maker and churn until the mixture has the consistency of softly whipped cream.
Quickly spoon into a plastic freezer box and stir in the crumble pieces.
Put on the lid, and then freeze for a minimum of 2 hours, or until the ice cream is firm enough to serve.
To make without an ice-creammaker, freeze the mixture, without the crumble, in the box for 3 to 4 hours,then whisk and return to the freezer.
Refreeze for a further 2 hours, then whisk again, and stir in the crumble before the final freezing.