Prepare the Streusel Topping – In a bowl, mix flour, brown sugar, and cinnamon. Stir in melted butter until crumbly. Set aside.
Make the Apple Filling – Toss apple slices with brown sugar, cinnamon, nutmeg, and lemon juice. Set aside.
Mix the Cake Batter – In a large bowl, cream butter and sugar until fluffy. Add eggs and vanilla extract, then gradually mix in flour, baking powder, salt, and milk until smooth.
Assemble the Cake – Spread half of the batter into a greased 9x13-inch baking pan. Add the apple filling, then spread the remaining batter on top. Sprinkle with streusel topping.
Bake – Preheat oven to 350°F (175°C) and bake for 45–50 minutes, until golden brown and a toothpick inserted comes out clean.
Cool & Serve – Let cool before slicing. Serve warm or at room temperature.