Preheat the oven to 375 degrees. Spray a 13x9 baking dish with cooking spray.
Prepare the filling: In a large saucepan, combine the chunks of berries, sugar, lemon juice, corn starch, and water. Stir to combine. Bring to a gentle boil over med-high heat. Once it starts to boil, reduce the heat to medium and simmer, stirring occasionally, until the liquid turns into a thick syrup (about 6-8 minutes). Remove the pan from the heat and set it aside.
For the crust/crumble: Using a food processor, hand mixer (or your hands), mix together flour. sugar, egg, baking powder, salt, and vanilla. Slowly add 1/2 cup cold (8 tablespoons) butter mixing until combined. Set aside in a small bowl 1 1/2 cups of the mixture for the topping.
Pour the remaining mixture into the baking dish packing it firmly.
Add to the reserved topping mixture, the brown sugar and oats, and the additional 2 tablespoons of cold butter. Mix until crumbly
Spread the strawberry filling over the crust and sprinkle the topping over the filling.
Bake for 40 minutes the filling will be bubbly and the crust will be golden.
Allow cooling for 1-2 hours to set up before slicing.