1.
Preheat the oven to 375 °F.
2.
Combine the butter, brown sugar and flour, and cream the mixture until it has a sandy texture. Add the oats and mix again to combine. Divide the mixture into three portions. Press one-third into the bottom of a baking pan, lined with parchment paper. Set aside the other two-thirds.
3.
In a bowl, combine the strawberries with the cornstarch, sugar,
vinegar and the juice and zest of the lemon. Stir well and spread the fruit mixture over the bottom crust. Then sprinkle the rest of the crumble topping over the top.
4.
Bake at 375 °F for 45-50 minutes or until the top is golden brown.
5.
Let it cool to room temperature before setting it in the fridge for a few hours or overnight.
6.
When the dessert is completely cool, cut it into squares before serving.