Preheat the oven to 350° then butter an 8 x 8 baking dish and set it aside.
Slice the strawberries then add all the berries to a medium size bowl.
Sprinkle on the sugar, lemon juice and corn starch over the berries and gently combine then transfer the fruit to the prepared baking dish.
Melt the butter in a medium size microwave safe bowl then add the flour, oats, sugar, butter, cinnamon and a pinch of salt. The mixture should be crumbly.
Using your hands, sprinkle the topping over the berries.
Bake the crisp 30-35 minutes or until the berry juices are bubbling on the edges and the topping is lightly browned.
Remove from the oven, cool 10 minutes then serve plain or with ice cream.