Peel apples, quarter, remove cores, and use a paring knife to thinly slice lengthwise (see photo).
Add to a large mixing bowl and top with remaining filling ingredients. Toss to combine. Add to a 9x13 (or similar size // as original recipe is written // use smaller or larger is altering batch size) baking dish.
Rinse and wipe out your mixing bowl and add all topping ingredients. Stir to combine, then use fingers to break down any clumps of muscovado sugar. Pour over apples in an even layer.
Bake for 50 minutes to 1 hour (uncovered) or until the filling is bubbly, the apples are very fork tender (especially in the center of the dish), and the topping is deep golden brown.
Best when fresh, though leftovers keep covered in the refrigerator up to 3 days, or in the freezer up to 1 month. Reheat in the microwave or in a 350-degree F (176 C) oven until warmed through.
*To keep this recipe gluten-free, ensure your oats are gluten free and sub the unbleached all-purpose for a gluten-free flour blend, such as my DIY gluten-free blend or Bob’s Red Mill Gluten-Free 1:1 Baking Flour. *Nutrition information is a rough estimate calculated without additional toppings. *Recipe adapted from Food 52.