For the crumble:
For the raspberry filling:
1. Preheat oven to 350F (175C). Line a 8-inch (20cm) square baking pan with a strip of parchment paper. Set aside.
2. Make the crumble: In a large bowl, combine oats, flour, salt, brown sugar and baking soda. Add the butter and mix until crumbly.
3. Make the filling: in a large bowl mix raspberries, lemon zest, sugar, cornstarch and vanilla extract. Set aside.
4. Press 2/3 of the crumble mixture into the bottom of the baking pan. Spread the filling over the base, Sprinkle the remaining oat mixture.
5. Bake for 35 minutes, until golden brown. Let cool completely, then refrigerate for at least one hour before cutting into squares.