Preheat oven to 350°F, 175°C or gas mark 4 and line the 9 x 13 inch baking dish with parchment paper. Set aside.
In a mixing bowl combine strawberries, sugar, cornstarch, lemon juice, and salt and toss the strawberries to evenly combine them.
In a heavy bottomed sauce pan add the strawberry mixture and heat to medium.
Stir frequently until strawberries release their juices and increase the heat if needed to bring to a boil. Cook until the mixture has thickened. Continue to boil, constantly stirring, for about a min but not too much longer than 2.
Remove the pan from the heat and allow mixture to cool.
For Crumble
In a food processor add the flour, both sugars, baking powder, cornstarch, and salt. Pulse until it is evenly mixed. Add the butter, lemon juice, eggs, and vanilla and pulse until the butter has been well incorporated. The mixture will look and feel dry this is ok.
The dough should hold its shape when you squeeze together but just barely.
Pour approximately ⅔ of the crumble mixture into your prepared pan and evenly press the crumble mixture into your pan. You want to make sure it is packed enough that it sticks together.
Using a spatula smooth the strawberry mixture evenly across the bottom layer. Then sprinkle the remaining crumble on top.
Bake for 50-60 minutes or until the top turns golden brown.
Allow to cool before slicing.
Notes
You can use a pastry blender but it will take considerably longer to blend this way. The dough is going to closer to the dry side.