Preheat oven to 350. Toss sliced apples with sugar, flour, 1 tsp cinnamon, and nutmeg. Pour apples into a 9x13 pan and spread evenly.
In a medium bowl, combine oats, flour, brown sugar, baking powder, baking soda, 1/2 tsp cinnamon, and 1/2 c melted butter. Mix until combined. Use hands to spread crumbles evenly over apple slices.
Bake for 45 minutes, or until bubbly and browned around the edges.
Serve warm with vanilla ice cream.