I decided to try using dark brown sugar which caramelizes very nicely with the butter and heat.
120g plain flour
50 g caster sugar
50 g dark brown sugar
75 g unsalted butter at room temperature
Actually the butter doesn't have to be at room temperature because it warms up as you start mixing the ingredients together. I recommend mixing it all by hand especially if you're using butter straight from the fridge. Keep mixing until you get a breadcrumb texture. Then place on top of your fruit mixture. I decided to do another apple and rhubarb mixture which I posted earlier.
You can see the nice caramelization around the edges of the dish. I topped it off with a scoop of vanilla ice cream to cool it down. Enjoy!