Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan to prevent the cake from sticking. Getting this ready first ensures everything comes together quickly.

In a large bowl, combine the yellow cake mix, water, vegetable oil, and eggs. Mix everything together until the batter is smooth and well combined. Make sure there are no lumps in the batter for the best texture.

Pour the prepared cake batter evenly into your greased and floured 9x13 inch baking pan, ensuring an even layer across the bottom.

In a separate bowl, toss the peeled and sliced apples with brown sugar, ground cinnamon, and lemon juice. Stir until the apples are thoroughly coated in the sugar and spice mixture. I like to gently toss everything by hand to make sure every apple slice gets some cinnamon-sugar flavor.

Spread the coated apple mixture evenly over the cake batter in the pan, covering the entire surface.

In another bowl, combine the all-purpose flour, brown sugar, and salt. Add the cold, diced butter and use your fingers or a pastry cutter to blend it in until the mixture looks like coarse crumbs. For the best crumbly texture, try not to overwork the butter—small, cold bits make the topping extra crisp.

Sprinkle the streusel topping evenly over the apples in the pan. Bake in the preheated oven for 50-60 minutes, or until the topping is golden brown and a toothpick inserted into the cake comes out clean. I recommend rotating the pan halfway through for even baking.

Remove the cake from the oven and allow it to cool in the pan before serving so the layers can set nicely. If you like, serve warm with a scoop of vanilla ice cream or a drizzle of caramel sauce.