Preheat the oven to 180°C (355F).

Combine all the ingredients in a bowl and rub the butter into the flour using your hands, so that the mixture comes together to form small, pea-sized balls of dough.

Sprinkle the crumble evenly into a baking tin and bake for 10 minutes at 180°C (355F). Remove from the oven and let cool to room temperature.

Cut the rhubarb into 1 cm (½ inch) lengths and place them in a large, shallow baking dish along with the sugar and lemon juice. Macerate for 30 minutes so that the rhubarb can release as much juice as possible.

Bake the rhubarb for about 25-30 minutes or until it's completely cooked. Remove from the oven and let cool. 

Drain the rhubarb juice. Make sure to save the juice as you will use it later.

Pour the rhubarb into a food processor or blender and process until you have a smooth puree. Pour it into a measuring jug, cover and transfer to the fridge to chill.

Place the rhubarb juice into a small pan and cook on a medium heat until it thickens and becomes a syrup. Remove from the heat and let cool. 

Combine the rhubarb juice and rhubarb puree.

Stir the cream into the rhubarb mix, pour into an ice-cream maker and churn until the mixture has the consistency of softly whipped cream. 

Quickly spoon it into the 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.