Preheat the oven to 180C/350F.
Mix the oats, flour, brown sugar, baking powder, cinnamon, and salt. Add melted butter and mix to form a crumbly topping.
Hull the strawberries, halving larger berries and leaving small ones whole. Toss with cornstarch.
Add white sugar, water, and vanilla to the strawberries and mix well.
Pour the strawberries into a baking pan and scatter the topping over them, keeping some chunky pieces.
Bake for 30 to 40 minutes, until the topping is deep golden and the strawberry syrup bubbles energetically around the edges.
Let the crumble stand for a few minutes, then serve with vanilla ice cream, cream, or custard.