Preheat the oven to 350F (180C). Line an 8 inch square baking pan with parchment paper.
In a large bowl, add the flour, oats, sugar and lemon zest. Mix to combine. Add the melted butter and mix with a spatula until you get a crumbly mixture.
Press about ⅔ of the mixture into the prepared pan. Press down to the bottom with a spoon or cup. Place in the freezer the remaining dough.
In a separate bowl, add the sliced strawberries, with lemon juice and cornstarch. Toss to combine. Spread on top of the crumble base and level.
Top with the remaining crumble, pinching the dough between your fingers to get some bigger crumble pieces.
Bake at 350F for about 45 minutes, or until lightly golden on top. Cool down completely before cutting into pieces. You can make 9 bigger or 16 smaller squares or triangles. Store in the refrigerator.
