Preheat the oven to 190C.
Start by cutting your butter (ideally super cold) into cubes. Add it with the flour to a large mixing bowl and lightly rub together with the tips of your fingers until you form a breadcrumb-like consistency.
Add the oats, sugar and a pinch of salt and mix to combine.
Pour 3/4 of the crumble mixture into the base of a lined baking tray. Press down until the mixture feels firm.
Chop the rhubarb into small slices and place on top of the crumble mixture, in a single layer.
Add dollops of jam across the top, over and between the slices of rhubarb.
Finally, scatter the remaining crumble mixture on top.
Bake in the oven for approx. 20 minutes until the fruit is soft and the crumble bars and golden.
It's a good idea to score the lines to form bars before the mixture cools completely, so it's easier to cut.
Allow to cool completely, before slicing into bars and enjoying with a cuppa!