Have your lemon curd ready and chilled before beginning the cookie dough.

Preheat the oven to 350 F. Spread the pistachios on a small cookie sheet and toast for 5-7 minutes, until fragrant. Set cool on the counter until completely cool. Grind the pistachios in a food processor until you have fine crumbs (pulse it to ensure it doesn’t turn into a paste!)

In a large frying pan, add the oats. Set over medium heat on the stove. Warm until hot and fragrant, tossing for even heating. Set aside to cool. Divide into 200g (1 ¾ cups) and 30g (¼ cup); the greater amount is for the bottom of the bar, the lesser will be for the topping.

Set the butter in a large heat safe bowl and warm until melted (60 seconds in the microwave should do it). Add the brown sugar, salt, and cinnamon, and whisk well to combine.

Whisk in the egg then switch to a rubber spatula and add the flour and baking soda, stir until smooth. Add the reserved greater amount of oats (200g or 1 ¾ cup) as well as most of the ground pistachios (the remainder will go into the crumble topping) and stir to combine.

Prepare a square baking pan, 9x9 or 8x8 (the latter will make thicker bars) by greasing it then lining with parchment paper (two overlaying sheets). Have the oven at 350 F for baking.

Press ⅔ of the oat mixture into the bottom of the pan, it should be flat and even. Pour the lemon curd on top and smooth into an even layer.

To the remaining oat mixture add the remaining oats (about ¼ cup) and (ground pistachios a few tablespoons). Use your hands to work it together until combined. In small bits, crumble/sprinkle the topping over the lemon curd, spreading it all over and leaving only small gaps.

Bake the bars until golden all over, for about 25-30 minutes. Let cool completely then transfer to the fridge to chill for a few hours. You can cut them before chilling, but the lemon curd will be a bit messy. Store leftovers in the fridge.