Preheat oven to 375 degrees.

Add the butter, brown sugar, egg, and vanilla to a large bowl. Mix with a handheld mixer on medium speed for 1-2 minutes or until combined.

Add the flour, baking powder, and salt to the bowl and mix until combined.

Mix in the oats using either a spatula or on low speed with the mixer until everything is well incorporated. The mixture may be a little thick and crumbly at this point depending on how soft your butter was.

Add the berries to a separate large bowl and sprinkle with the cornstarch and sugar. Toss to combine.

Pour in the lemon juice and add the zest. Toss until the berries are well coated.

Press half of the oat crumble into the bottom of a 9x9 inch baking dish that has been sprayed with non-stick cooking spray. Spread the berries out over the crust in a single layer and then sprinkle the remaining crumble over the berries.

Bake in the preheated oven for 40-45 minutes or until the crust is golden brown.

Let rest for at least 3-4 hours or until it reaches room temperature. It’s best if served within the first 24 hours. After this time it can still be eaten for up to 2-3 days, but the crust will get more soggy.