Preheat your oven to 350°F (180°C). Grease a 9x13 inch baking pan to prevent sticking.

In a large mixing bowl, combine the yellow cake mix, water, vegetable oil, and eggs. Mix well until you have a smooth batter, then pour it evenly into your prepared baking pan.

In another bowl, take your sliced apples and toss them with 3 tablespoons of brown sugar, cinnamon, and lemon juice. Make sure the apples are well coated, then spread the apple mixture evenly over the cake batter.

Now, in a separate bowl, mix the all-purpose flour, ½ cup of brown sugar, and salt. Add in the cold, cubed butter, and use your fingers or a pastry cutter to blend everything until it reaches a crumbly texture. Sprinkle this crumb mixture generously over the apples.

Bake in the preheated oven for 35 to 60 minutes. You’ll know it’s done when a toothpick inserted in the center comes out clean.

Once baked, allow the cake to cool completely before serving. If you like, dust it with powdered sugar for an extra touch of sweetness.