Toss berries with sugar, cornstarch, lemon juice, zest, and vanilla. Pour into a 9x9 dish.
Mix oats, flour, brown sugar, and cinnamon.
Cut in cold butter until crumbly. Sprinkle over berries.
Bake at 375°F for 40 mins until bubbling and golden.
Let rest for 15 mins before serving to allow juices to thicken.
Notes
Use a cast-iron skillet for extra-crispy edges.Don't over-sweeten the berries; let their natural tartness shine.Add chopped pecans to the topping for a nutty crunch.Serve warm for the best experience.