In a medium bowl, mix together the oats, flour, brown sugar, and cinnamon.Cut butter into pieces and mix into the oat mixture until the butter is evenly distributed throughout the oats and in pea-sized pieces. Set the crumb mixture aside.
Peel and core apples. Slice into ¼-inch thin slices.
Heat butter in a 10-inch cast iron skillet over medium-low heat.
Add the apples to the butter. Sprinkle the remaining ingredients over the top of the apples, then toss to coat. Cook apples over medium heat for about 3 minutes to caramelize.
Spoon the crumb topping over the apples. Bake for 30-40 minutes, or until the apples are soft and the top has started to brown.Let rest for 10 minutes, then serve with ice cream.
It is important to use cold butter so that your crumb topping is not mushy. I recommend a mix of sweet and tart apples. See apple varietal recommendations in the post.Storage: Cover and refrigerate for up to 5 days. Baking without a cast iron skillet: Toss the filling ingredients together and place in a 3-quart baking dish. (You do not need to cook the apples first.) Top with the streusel topping and bake at 350°F (177°C) for 40-50 minutes.Variations: Use your favorite mix of fruit, like blueberry-apple or pear. A total of 6 cups of fruit is needed. Add up to 1 cup of chopped nuts to the streusel.