• Preheat the oven to 350ºF (180ºC).
• Butter or spray a 9x13-inch (23x33cm) rectangular pan. Line with a piece of parchment paper, covering the pan's bottom and two long sides. The two short sides will remain unlined but greased. This will help you remove the cake from the pan. 

For the crumble:

• In a small bowl, combine ½ cup light brown sugar, 2 tablespoons all-purpose flour and 1 teaspoon ground cinnamon.
• Add 2 tablespoons unsalted butter and mix with a fork until they are small and somewhat mixed.
• Stir in 1 cup walnuts and combine. Refrigerate while preparing the cake. 

For the apples:

• Peel, core, and dice 1 pound baking apples. Toss them with 1 tablespoon sugar and ½ teaspoon ground cinnamon. Reserve.

For the cake batter:

• In a large bowl (stand or hand-held electric mixer), beat 1 cup unsalted butter with ½ teaspoon lemon zest for 1 minute.
• Add 1 cup white sugar and ¾ cup brown sugar gradually and beat until light and creamy, about 3 minutes.
• Add 3 eggs, one at a time, beating well to incorporate each.
• Add 1 teaspoon vanilla extract and mix.
• Sift the flour mixture (2 ¾ cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda and ¾ teaspoon salt) before adding it, or have it measured and sift directly over the butter mixture when instructed.
• Add them in 3 parts, alternating with ¾ cup sour cream and ¼ cup whole milk in 2 parts, beginning and ending with the dry ingredients. Mix well, but don’t overbeat. The mixture will be thick.
• Spread half of this cake batter in the prepared pan, spreading to cover it.
• Top it with the apples, scattering them over the surface.
• Cover with the remaining half of the batter, spreading carefully over the apples. Add the cake batter in mounds so they'll be easier to spread. And aid yourself with the back of a spoon barely dipped in water. You will find that the batter glides more easily, and the apples don't stick as much.
• Sprinkle the crumb or streusel topping over the batter. It will not cover it completely, and that is fine. 
• Bake for 40-45 minutes, or until a cake tester or toothpick comes out clean. Don’t overbake it. The top will be golden brown, and the topping will have seeped a little into the cake. If it darkens too quickly, tent with aluminum foil and continue baking. 
• Let cool on a wire rack.
• Remove from the pan with the aid of the parchment paper.
• Drizzle the glaze (below) over it, letting it dry before cutting and eating at room temperature.
• Keep covered in plastic wrap or an airtight container for no more than a day at room temperature. After that, keep refrigerated for a few days and up to a week. I recommend eating it at room temperature or warming it up a little before eating.

For the glaze:

• Mix 1 cup powdered sugar and 2 to 3 tablespoons lemon juice in a small bowl until smooth. It should be like thick honey. 
• Drizzle on the cooled cake and let dry before cutting. Refrigerate leftovers, well covered.