Cinnamon Apple Crumb Sheet Cake, Blueberry Layer

Steps

1. Make the blueberry filling first: in a small saucepan, stir the frozen blueberries, half of the granulated sugar, the cornstarch and the lemon juice. Cook over medium heat, stirring, until bubbling and thick, about 8 minutes. Spread on a plate and let cool while you continue.
2. Preheat the oven to 350°F (175°C). Line a 9x13-inch pan with parchment, leaving the sides overhanging.
3. Make the crumb topping: in a small bowl, mix one-eighth of the flour (about 40 g), one-fifth of the brown sugar (about 50 g), one-seventh of the cinnamon (about 1 g), the oats and the walnuts. Stir in the melted butter until clumpy, then set aside.
4. Make the dry mix: in a medium bowl, whisk the remaining flour, the baking powder, the baking soda, the salt, the nutmeg and the remaining cinnamon.
5. Make the wet mix: in a large bowl, whisk the eggs, the remaining brown sugar and the remaining granulated sugar until smooth, then whisk in the applesauce, canola oil and vanilla.
6. Fold the dry mix into the wet mix just until no dry streaks remain, then fold in the diced apple.
7. Spread half of the batter in the pan. Dollop the cooled blueberry filling over it and spread gently, leaving a 1/2-inch border. Spoon the remaining half of the batter over the top and smooth it to cover the filling.
8. Scatter the crumb topping evenly over the batter.
9. Bake 45 to 50 minutes, until the crumb is golden and a toothpick inserted into the cake (not the berry layer) comes out clean.
10. Cool the cake in the pan on a rack for at least 1 hour so the blueberry layer sets, then lift it out by the parchment and cut into 12 slices.