I think the best kind of crumble mix is with oats in it, it just goes nice and crunchy and...well...crumbly :P. And perhaps some shredded/dessicated coconut.
I know how you feel about the availability of rhubarb Dolphinesque. In winter its like AU$5 a bunch, which is when you want it most. So in the summer when its really dirt cheap, I wash the stalks, dry them thouroughly cut them into 2.5cm/1 inch bits then freeze them in a single layer on a tray then put them into freezer bags or sealable containers. Works great!
This one is interesting, the passionfruit and rhubarb go well together even though it isn't a regular pairing of flavours. I prefer to stew my fruit a bit before baking to make it go that little bit extra soft, but there are those that prefer a bit of fruity crunch still. To each their own.
Rhubarb and Passionfruit Crumbles
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6 stalks rhubarb, chopped
1/3 cup passionfruit pulp
250g strawberries, hulled and halved
1/4 cup caster sugar
1/2 cup brown sugar
1 1/3 cups rolled oats
1/2 cup plain flour
90g butter, softened
1 tsp ground cinnamon
Preheat oven to 160'C. Combine rhubarb, passionfruit, strawberries and sugar in a bowl and divide between 6x1cup capacity ramekins or one large dish.
To make the topping combine brown sugar, oats, flour, butter and cinnamon. Spoon topping on to the top of the fruit and bake for 30 minutes or until the top is golden and the fruit is soft.