Preheat the oven to 350°F (175°C). Lightly grease a 9x9” or similar-sized baking dish.
Prep the apples: In a large bowl, toss apple slices with lemon juice, sugar, cinnamon, nutmeg, and flour. Spread evenly into the prepared baking dish.
Make the topping: In another bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add cold butter cubes and cut in with a pastry blender or your fingers until the mixture resembles coarse crumbs.
Assemble & bake: Sprinkle the topping evenly over the apples. Bake for 40–45 minutes, or until the top is golden brown and the apples are bubbling.
Cool slightly before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream—because why not?