1. Preheat the oven to 350ºF / 180ºC.
2. 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:

1. In a small bowl, mix sugar, flour, and cinnamon.
2. Add pieces of butter and mix with a fork until they are small and all is somewhat mixed.
3. Add nuts and mix.

For the apples:

1. Peel, core, and dice the apples.
2. Mix the pieces with the tablespoon of sugar and cinnamon. Reserve.

For the cake batter:

1. In a large bowl (stand or hand-held electric mixer), beat butter with lemon zest for 1 minute.
2. Add sugars gradually and beat until light and creamy, about 3 minutes.
3. Add eggs, one at a time, beating well to incorporate each.
4. Add vanilla extract and mix.
5. Sift the flour mixture (flour, baking powder, baking soda, and salt) before adding it, or have it measured and sift as you add to the butter mixture.
6. Add them in 3 parts, alternating with the sour cream and milk in 2 parts, beginning and ending with the dry ingredients.
7. Mix well but don’t overbeat. The mixture will be thick.
8. Spread half of this cake batter in the prepared pan, spreading to cover it.
9. Top it with the apples, scattering them over the surface.
10. 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.
11. Sprinkle the crumb topping over the batter. It will not cover it completely, and that is fine. 
12. 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. 
13. Let cool on a wire rack.
14. Remove from the pan with the aid of parchment paper.
15. Drizzle the glaze (below) over it, letting it dry before cutting and eating at room temperature.
16. Keep covered in plastic wrap or an airtight container for a day at room temperature. After that, keep refrigerated.

For the glaze:

1. Mix powdered sugar and 2 tablespoons of lemon juice in a small bowl until smooth. 
2. Add more juice if necessary. It should be like thick honey. 
3. Drizzle on cooled cake. 
4. Refrigerate leftovers, well covered.