Preheat your oven to 350°F (175°C).
Grease a 9x9-inch baking dish with butter or nonstick spray.
In a large bowl, toss the sliced apples with granulated sugar, cinnamon, vanilla, and 1 tablespoon flour. Spread them in the baking dish.
In another bowl, combine brown sugar, oats, flour, salt, and cold butter. Use a pastry cutter, fork, or your fingers to blend until it looks like coarse crumbs.
Sprinkle the topping evenly over the apples.
Bake for 40–45 minutes, until the topping is golden and the apples are tender.
Serve warm, ideally with vanilla ice cream or whipped cream.