Line a baking tin with baking paper and preheat your oven to 180C/350F.
Toss the strawberries, sugar and cornstarch into a saucepan on medium heat and simmer until thickened and reduced, approximately 10 minutes.
Meanwhile, toss the butter, sugar, ground cinnamon and vanilla extract in a bowl and whip until pale and fluffy, approximately 3 minutes.
In a separate bowl, stir the flour, rolled oats and baking powder until combined.
Toss the dry ingredients into the wet ingredients and stir until the mixture resembles a rough crumble.
Spoon ½ the oat crumble mixture into your baking tin and and press firmly until even.
Top with the strawberry jam mixture and spread evenly before crumbling over the remaining oat mixture.
Place in the oven and bake until golden, approximately 25 minutes.
Remove and set aside to cool before removing from the tin.
Slice and serve.
Dig in!