Preheat oven and grease skillet. Begin by preheating your oven to 350°F (177°C). Grease your cast iron skillet. If using butter sticks, use the wrapper from the butter to grease the skillet, as there is always some excess butter stuck to it. Otherwise, use a pat of butter. Set aside.
Melt butter. In a small microwavable bowl or small saucepan, melt the butter. Set aside.
½ cup unsalted butter
Combine dry ingredients. In a medium bowl, combine the flour, oats, brown sugar and salt.
1 cup all-purpose flour, 1 cup rolled oats, 1 cup dark brown sugar, ¼ teaspoon table salt
Whisk to combine. Using a small whisk, combine the dry ingredients.
Pour in melted butter. Pour the melted butter over the combined dry ingredients.
Stir the butter to combine. Stir butter into dry ingredients until the dry ingredients have become fully hydrated with the butter. I find using my hands is easier than using a spatula.
Peel, core and slice the apples. Peel, core and slice the apples into ¼-inch thick slices (approximately 5 centimetres).
1 ½ pounds (roughly 5 medium) apples
Add apples to the skillet. Add the sliced apples to your cast iron skillet and arrange them in an even layer.
Add cinnamon. Sprinkle cinnamon evenly over the apples.
¼ teaspoon ground cinnamon
Add blackberries. Scatter the blackberries evenly over the apples.
12 ounces fresh blackberries
Add crumble mixture and bake. Finally, add the crumble mixture to the skillet to form an even layer over the fruit. Bake uncovered on the bottom rack of a preheated 350°F (177°C) oven for 45 minutes. The Apple Blackberry Crumble is done when you insert a pairing knife into the centre of the skillet, and there is no resistance.