Combine 70g butter (cold and cubed), 125ml brown sugar, 125ml flour, 125ml oats, 125ml chopped walnuts and a pinch each of cinnamon and salt in a food processor and pulse to a coarse meal. 

In a greased baking dish place 300g blueberries, 2 apples (peeled and cut into bite-sized pieces), 2 rhubarb stalks (sliced into small pieces). Sprinkle on 2tbs sugar and 2tsp cornstarch. Top with the crumble. 

Bake in the oven at 175c till the crumble turns golden and the filling starts to bubble over (around 30-40 min).

Serve with a dollop of clotted cream.

Serves 4.