Add chopped fruit to a medium bowl and toss with sugar, lemon juice, cornstarch and salt. Pour into a ceramic or glass pie dish and set aside.
Place flour, oats, walnuts, brown sugar and spices into a medium bowl and mix to combine. Cut chilled butter into pats and work into dry mix with your hands until fully incorporated. Crumble over the surface of the strawberry rhubarb filling, then sprinkle with a liberal pinch of flaky sea salt.
Bake crumble for 55-65 minutes until crumble topping has browned and your fruit filling is bubbling all over. Let cool completely.