Preheat the oven to 350°F (175°C). Lightly grease a 10¼-inch springform pan, or line a standard cake pan with parchment paper.
In a small bowl, mix the blueberries, coconut sugar, lemon juice, and melted coconut oil (or applesauce). Spread this mixture evenly over the bottom of the prepared pan.
3 cups fresh or frozen blueberries, 3 tablespoons coconut sugar or brown sugar, 1 ½ tablespoons lemon juice, 1 ½ tablespoons melted coconut oil or unsweetened applesauce
In a separate small bowl, mix the ground flaxseed with water to make flax eggs. Let it sit for 5 minutes to thicken.
1 ½ tablespoons ground flaxseed, 4 tablespoons water
In a large mixing bowl, whisk together the white whole wheat flour, oat flour, baking powder, cinnamon (if using), and salt.
1 ½ cups white whole wheat flour, ¾ cup oat flour, 3 teaspoons baking powder, ¾ teaspoon cinnamon, ½ teaspoon salt
In another bowl, combine the almond milk, maple syrup, applesauce, melted coconut oil (if using), vanilla extract, lemon zest (if using), and the thickened flax mixture.
6 tablespoons maple syrup, 6 tablespoons unsweetened applesauce, 1 ½ cups unsweetened almond milk or any plant milk, 3 tablespoons melted coconut oil or more applesauce, 1 ½ teaspoons vanilla extract, Zest of 1 lemon
Pour the wet ingredients into the dry ingredients. Stir gently until just combined. Do not overmix. A few small lumps are okay.
Pour the batter evenly over the blueberry mixture in the pan. Smooth the top with a spatula.
Bake for 35 to 45 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool in the pan for 10 to 15 minutes. Then carefully flip it onto a serving plate so the blueberry topping is on top.
Slice, serve, and enjoy!