Preheat the oven to 350°F (180℃). Lightly grease a 9x5-inch loaf pan.
Make the apple filling. In a mixing bowl, toss together the apples, flour, brown sugar, cinnamon, and lemon juice until the apples are evenly coated. Spread the apple mixture evenly in the loaf pan. 2 ½ cups apples 1 tablespoon all-purpose flour 2 Tablespoons packed light brown sugar ½ teaspoon ground cinnamon ¼ teaspoon lemon juice
Make the oat topping. In the same bowl, stir together the flour, oats, brown sugar, cinnamon, and salt. Pour in the melted butter and vanilla and stir until the mixture is evenly moistened and crumbly. 6 Tablespoons (75 g) all-purpose flour 6 Tablespoons (75 g)old-fashioned oats ½ cup (107 g) packed light brown sugar ¼ teaspoon ground cinnamon ¼ teaspoon salt 4 Tablespoons (1/2 stick, 57 g) unsalted butter ½ teaspoon vanilla extract
Assemble. Crumble the oat topping evenly over the apples. Don't press the topping down—you want to keep it loose and crumbly so it gets crisp in the oven.
Bake. Bake at 350°F for about 30–35 minutes, or until the topping is golden brown and you can see the apple filling bubbling around the edges. If the topping starts getting too brown before the apples are tender, loosely cover the pan with foil.
Cool and serve. Let the apple crisp cool for about 10–15 minutes before serving. It's especially good warm with vanilla ice cream or whipped cream.