• Preheat the oven to 350 degrees F 30 minutes before baking.
• Spray a 9x5 loaf pan with cooking spray and line the bottom with parchment. Spray again with cooking spray.

Topping

• In a food processor, pulse the sugars, nuts and cinnamon until the nuts are coarsely chopped.
• Reserve 1/2 cup of this mixture to use with the filling.
• Add the flour, butter and vanilla to the remainder and pulse briefly just until the butter is absorbed.
• Empty it into a bowl and refrigerate for about 20 minutes to firm up, then use your fingertip to form a coarse, crumbly mixture for the topping.

Filling

• Peel and core the apple. Cut it into 1/4 inch thick slices, and toss with lemon juice. Set aside.
• In a medium bowl, lightly combine the egg, the yolks, and about one fourth of the sour cream, and the vanilla.
• In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, baking powder, baking soda, and salt. Mix on low speed for 30 seconds to combine.
• Add the butter and the remaining sour cream and mix until the dry ingredients are moistened.
• Increase the speed to medium and beat for 1 minute.
• Scrape down the sides of the bowl.
• Gradually add the egg mixture in two batches, beating for 20 seconds after each addition to incorporate the ingredients.
• Fill your prepared loaf tin with 2/3 of the batter. Smooth the surface.
• Sprinkle with the reserved 1/2 cup crumb mixture. and top with the apple slices, arranging them in two rows of overlapping slices.
• Drop the remaining batter in large blobs over the fruit and spread it evenly, preferably with an offset spatula. The batter should be about 3/4 of an inch from the top of the tin.
• Sprinkle with the crumb topping.
• Bake the bread for 50-60 minutes or until a wooden toothpick inserted in the center comes out clean. Tent loosely with buttered foil after 45 minutes to prevent overbrowning.
• Cool the bread in the tin on a wire rack for 10 minutes. Carefully remove the bread from the tin and allow to completely cool on a wire rack. Store in an airtight container.