Preheat the oven to 350°F. Spray an 11x7-inch baking dish with cooking spray and set aside.
Remove the stems from the strawberries. Prepare them by leaving small ones whole, cut medium ones in half, and quarter large ones.
Place the strawberries in a large bowl. Toss them until coated in the cornstarch, sugar, lemon juice, and vanilla. Set aside.
In a medium-sized bowl, stir together the oats, flour, brown sugar, melted butter, and cinnamon until combined and crumbly.
Give the strawberries another good stir and place them in the baking dish in an even layer.
Take your hands and add the crumble on top, breaking it up and crumbling it evenly all over the strawberries.
Bake for 30-40 minutes until the strawberry mixture is bubbling up along the sides of the baking dish, and the crumble is golden brown.
Allow the crumble to sit for 10 minutes before serving. Don't skip this step. It allows the filling time to thicken up. Then scoop and serve!
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.
I don't recommend frozen strawberries (see my FAQ's above for more information on that).
This method will work for any berries.
Reheat by placing it back into the oven or the microwave until warm.
Keep leftovers in the fridge for up to 3 days, freeze for up to 3 months.
You can use a 9x9-inch baking dish, but put something under to help catch anything that spills over.
I wouldn't recommend using frozen berries, they'll end up mushy or make the filling runny. If you must use them, just be cautious.