For the dough:

1. Preheat the oven to 200 degrees Celsius or 180 degrees if using a fan oven. Grease and line a 20 x 20cm square baking tin with parchment paper, leaving a few centimetres of paper hanging over the sides.
2. Cut the butter into cubes, and add to a food processor with the oats, flour, sugar and salt. Pulse the ingredients until the mixture resembles coarse breadcrumbs. Alternatively, put into a large bowl and rub together with your fingertips.
3. Press two-thirds of the mixture into the prepared tin and pack together to form an even layer. Set aside the remaining third. Bake the base in the preheated oven for 10 minutes, then remove from the oven and set aside to cool slightly.
4. Reduce the oven temperature to 180 degrees Celsius or 160 fan.

For the filling:

1. Tip the blueberries into a large bowl and grate in the zest from the lemon. Halve the lemon and squeeze in the juice, then add the cornflour, honey, and jam. Gently toss everything together until the blueberries are completely coated.
2. Spread the blueberries on top of the baked base.
3. Add the almonds to the remaining third of the crumbly mixture and sprinkle this over the blueberries in clusters, leaving some of the blueberries visible under the topping.
4. Bake for 35-40 minutes, until the topping is golden brown and the blueberries are bubbling.
5. Remove from the oven and leave to cool completely in the tin. If you would like the bars to have a clean cut, refrigerate for about two hours before slicing into nine squares.
6. If you would like crumbly soft bars, slice once cool without refrigerating.
7. The bars will keep for three days in an airtight container in the fridge, or they can be frozen for up to one month.