Mix all the filling ingredients together in a bowl and evenly distribute to an oval casserole dish.
In a clean large bowl put all of the topping ingredients and work the butter in to pea sized granules with your fingers. You can also cut the butter in with a pastry cutter.
Layer the topping on the filling, evenly. Patting lightly to keep in place.
Place the casserole on a baking sheet, in case it bubbles over.
Bake 350°F for 50 mins to 1 hour. Until the centre is bubbling.
I find this can take over an hour on the times where I dome the topping. If I use a longer rectangle dish it can take around 45 mins. Start checking it after 35 mins in the oven. If using from the fridge let it come to room temp for a while, it will take longer to cook, otherwise the dish may crack and it can take much longer to cook.