This is the perfect apple crisp. It's easy to make and is loaded with a crisp oat-streusel topping and filled with cinnamon-spiced apples.
Preheat oven to 350° F (177 °C). Lightly grease a 9x13-inch baking pan with softened butter.
Prepare the Topping. Whisk together flour, brown sugar, cinnamon, and salt. Add the oats and stir to combine. Stir in the melted butter until there are no spots of dry topping in the bowl. Set the topping aside.
Prepare the Filling. Stir together the apple slices, lemon zest and juice. Add the brown sugar, cinnamon, and salt. Taste. Add more sugar if needed. Add the flour and stir until all the apple slices are evenly coated.