In a medium saucepan, add 3 cups of the blueberries, along with sugar, lemon zest, and vanilla over medium heat. Stir occasionally until it begins to boil.
In a small bowl, mix together the cornstarch and water well.
Add cornstarch mixture to saucepan and stir constantly until well incorporated and it starts to thicken. Remove from heat and set aside.
Preheat oven to 350 degrees F. Line a 9x13 baking dish with parchment paper.
In a large bowl add oats, flour, baking powder and salt. Stir together.
In a smaller bowl add applesauce, maple syrup, and vanilla. Stir together well.
Add the wet ingredients to the dry and stir together until combined well.
Add half of the mixture to your pan and press it down evenly across the pan.
Pour berry filling into the dish and spread evenly across. Evenly spread remaining cup of blueberries across.
Evenly add the rest of the oat mixture.
Bake for 30 minutes. Remove and set aside for at least 10 minutes to set up before cutting.
Store leftovers covered at room temperature for 3-5 days or in the fridge for up to 2 weeks. Heat for 10-15 seconds if desired to eat.