Heat oven to 180C/160C fan/gas 4 and line a 22cm square tin with baking parchment. Slice the strawberries thinly and mix well with 4 tsp lemon juice, then put them in a sieve over a bowl and set aside to macerate, discarding the liquid that drips into the bowl.

Put the flour, 175g caster sugar, 1 tsp cinnamon and 1/2 tsp salt in a food processor. Scrape in the seeds from the vanilla pod, then add the softened butter and pulse until the mixture comes together. Spoon out a quarter of the mixture into a mixing bowl and set aside. Tip the rest of the mixture into the bottom of your tin and press down firmly to make an even base. Prick a few times with a fork, then bake for 25 mins until golden.

While the base is cooking, add the cold butter, remaining sugar, the oats, hazelnuts, remaining cinnamon and the lemon zest to the reserved mixture. Use your fingers to rub together until crumbly.

When the base is done, mix the sliced strawberries with the jam and arrange on top. Sprinkle over the crumbly topping and bake for another 30-35 mins until the top is golden and the strawberries juicy and bubbling. Leave to cool completely in the tin before cutting into bars or squares.