Preheat the oven to 170 C. Lightly grease and line the baking tin with baking paper with plenty of overhang.
In a large bowl, mix together flour, oats, sugar and salt.
Pour the melted butter over the flour mixture and combine all together, until the mixture resembles the texture of the wet sand. You want a mix of fine crumbs and chunky bits.
Transfer ⅔ of the crumble mixture into the baking tin, and press it firmly into the tin to form the base of the crumble. Use flat object to smooth out the surface, a measuring cup or a back of the spoon will work well. Place the remaining crumble mixture into the freezer until you will need it.
Place chopped strawberries in a medium bowl and sprinkle 1 tablespoon of sugar over them, then gently stir together. Add 1 teaspoon of vanilla bean paste (optional) and stir again. Spread the strawberries on top of the crumble base in an even layer. Spread the strawberry jam on top of strawberries.
Remove the remaining crumble from the freezer and scatter it all over the strawberries in an even layer.
Bake for 35- 40 minutes until golden brown and bubbly. Leave in the baking tin to cool for about 30 minutes, then transfer to a fridge for at least an hour. Remove from the fridge and from the baking tin and slice into 9 equal squares. Enjoy!
Store any leftover bars in the air tight container in the fridge for up to 5 days.
To freeze, wrap the sliced bars in kitchen foil, place in the freezer-friendly container and transfer to the freezer for up to 3 months. Thaw at room temperature for about 2 hours.