Preheat oven to 400 degrees Fahrenheit. Prepare the pie crust in a deep dish pie pan and place on a baking sheet. Set aside.
In a large saucepan, add frozen berries and white sugar and heat over medium-high heat until lightly boiling. Mix lemon juice and cornstarch together and pour into the berries. Stir and then remove from heat. Add the butter and stir until fully melted. The berry filling will thicken as it cools.
In a medium bowl, stir together flour, oats, white sugar brown sugar, and salt. Add in the softened butter and mix with a fork or clean hands until the topping becomes crumbly. Pour the filling into the pie crust and add the crumble topping evenly over the entire top of the pie.
Place the pie pan with the baking sheet into the oven and bake for 25 minutes until the topping becomes golden brown. Remove from the oven. Place a piece of aluminum foil over the entire pie without pinching the sides and return to the oven. Bake for an additional 25-35 minutes until the bottom of the crust has fully cooked.
Remove from oven and allow to fully cool for around 3 or more hours before slicing. Enjoy!