Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan lined with parchment paper.

In a large mixing bowl, combine flour, oats, brown sugar, granulated sugar, salt, and half of the cinnamon. Cut in the butter until mixture resembles coarse crumbs.

Press the remaining mixture into the bottom of the pan. Bake for about 10–12 minutes until lightly browned.

Chop apples and toss in bowl with cornstarch, lemon juice, vanilla extract, remaining cinnamon, and optional nutmeg.

Spread the apple mixture over the warm crust and top with reserved crumb mixture.

Bake for an additional 25–30 minutes until the top is golden brown and filling is bubbly.

Allow to cool completely before slicing. Optionally, drizzle with glaze made from powdered sugar and milk.