Place blueberries in a saucepan. Add the water and agar flakes to the blueberry mixture and stir to combine. Bring this to a boil over medium-high heat. Once it begins to boil, reduce heat to medium and simmer for about 5 minutes, stirring occasionally. Remove from heat.
Pour the lemon juice and agave in the blueberries. Stir to combine. Transfer to a bowl and refrigerate.
Combine the oats, almond meal, flax meal, baking powder, and sea salt in a bowl. Stir to combine. Scoot the ingredients to one side of the bowl. In the other half of the bowl, combine the applesauce, agave nectar, almond butter, milk, vanilla, and vinegar. Stir to combine. Then stir the wet and dry ingredients together. Add a tablespoon or two of water if necessary.
Take 3/4 of that oats mixture and press evenly across the bottom of your prepared pan. Set the rest of the batter aside.
Place the crust in the oven and bake for 15 minutes.
Remove crust from the oven. Check the blueberry filling. It should have thickened by now. Spread it over the baked crust.
Place dollops of the remaining crust batter distributed equally over the blueberry filling. Return to the oven and bake for 10 minutes, until the top crust turns golden brown.
Remove from oven and set aside to cool before refrigerating to allow the filling to set completely.
You can substitute 1 tablespoon cornstarch for agar flakes.