Preheat oven to 350. Grease and flour a 9x13 pan and set aside.
Mix together flour, powdered sugar, and salt. Add cold, cubed butter. Using a fork or a pastry blender, cut butter into the dry ingredients until it starts to get crumbly. You can continue with the fork or pastry blender, but I find it easiest to mix the rest of the way with clean hands.
Evenly drop clumps of dough into the pan and then use your fingers to press down into the pan. Bake for 25-30 minutes, until the edges start to get golden. Meanwhile, prepare the filling and streusel topping.
Mix together the cornstarch and sugar, set aside.
Dice the strawberries into small pieces. Mix together with the sugar mixture, strawberry preserves, and lemon zest.
Spread evenly across the par-baked crust.
Combine flour, brown sugar, oats, and salt.
Add butter and cut in with a fork or pastry. Mix until the butter is about pea sized and clumpy.
Spread evenly across the top of the strawberry filling.
Bake for 30-35 minutes, until the streusel topping begins to brown.