Pulse 125g walnuts and 75g chopped dates together in a high powered blender or food processor.
Transfer to a mixing bowl and rub in 50g coconut oil or butter and 30g flour with your fingers.
Tip out onto a lightly floured surface and knead for a few seconds until it comes together into a dough.
Wrap in cling film and place in the fridge for 30 minutes.
Meanwhile place 400g rhubarb and the juice of half a lemon in a small saucepan and heat gently until soft and broken down.
Add 4 tbsp chia seeds and 3 tbsp maple syrup, stir well, set aside and leave to cool and thicken. You can put it in the fridge once cooled if you don't want to make the bars straight away.
Pre-heat the oven to 180°C (fan)/200°C/gas mark 6.
Grease and base line a 20cm square cake tin with baking paper. A loose-bottomed tin is ideal but if you don't have one leave a little bit of baking paper hanging over the edges to make it easier to remove the bars when they've been cooked.
Remove the dough from the fridge and roll out on a lightly floured surface. Press into the bottom of the tin.
Spread the rhubarb chia jam over the top and bake in the centre of the oven for 20 minutes.
Allow to cool in the tin then cut into slices and remove.