• Line a ¾ sheet pan with nonstick foil and brush the bottom with 1 tbsp or more of melted butter.
• Preheat your oven to 425°F / 218 ℃.

To make the streusel:

• In a small food processor/mini chopper, combine all the streusel ingredients.
  56 g butter, 55 g brown sugar, 40 g oats, 30 g white whole wheat flour, 30 g pecan pieces, ¼ tsp salt, ½ tsp cinnamon, 1/8 tsp nutmeg, Pinch allspice
• Pulse until the mixture comes together and resembles coarse crumbs. Do not overmix, it should be loose and crumbly, not a smooth dough or paste.
• Set aside in the refrigerator until assembly.

To make the cake batter:

• Whisk all the dry ingredients together in a large bowl.
  400 g white whole wheat flour, 50 g flaxseed meal, 80 g powdered sugar, 150 g cornstarch, 40 g baking powder, 15 g baking soda, 1 tsp ground cinnamon, 1 tsp fine sea salt
• In a medium bowl, combine the eggs, buttermilk, vanilla, and the oil, whisk to mix well.
  4 large eggs, 1000 ml buttermilk, 15 ml vanilla extract, 150 g oil
• Pour the wet ingredients into the dry ingredients and mix until just combined. Do NOT over-mix. Stop mixing while you can still see some small lumps in the batter.

To assemble for baking:

• Scrape the batter into the prepared sheet pan. Evenly spread the batter using an offset spatula. Gently shake the sheet pan to make sure the batter is evenly spread, but do not shake too hard or knock the pan on your work surface as this may deflate the batter.
• Spoon the apple pie filling over the batter in long even lines. Use a butter knife to swirl the apple filling through the batter.
  325 g apple pie filling
• Sprinkle the streusel mixture evenly over the entire surface of the cake.
• Place the sheet pan on a rack in the center of your oven and bake for 10-12 minutes. Check at 8-10 minutes, initially. The cake is done if the center is springy to the touch, and a toothpick inserted into the middle comes out with just a few moist crumbs attached.
• Remove the sheet pan from the oven and allow it to cool slightly.
• Cut the cake (while warm) into 12 (large), 24 (medium), or 48 (small) portions.