Warm 2 tablespoons of the olive oil and the knob of butter in a medium saucepan. Add the cumin seeds and cook for 30 seconds, until fragrant. Add the onion and cook on a medium–low heat for about 10 minutes, until translucent and lightly golden.
While the onion is cooking, tip the rice into a saucepan of salted water, bring to the boil and cook for 4 minutes, then drain and refresh in cold water.
Once the onions are cooked, remove the pan from the heat. Add the par-boiled rice, along with the allspice, Aleppo pepper, chopped tomato or tomato purée and half the parsley, and season generously.
Heat the air fryer to 180°C.
Make a slit down the length of a pepper, starting at the top and stopping about 2cm/ ¾ inch from the bottom, so that you have a pouch. Carefully remove the seeds and pith and rub with a little of the remaining olive oil. Fill the pocket with one quarter of the rice mixture and gently push the edges together. Repeat with the remaining peppers and rice mixture.
Get the peppers into the air-fryer basket, making sure there is space between each, and cook for 20–30 minutes, until beginning to char.
Sprinkle the peppers with the toasted walnuts, the remaining parsley and a big sprinkling of Aleppo pepper. Serve individually or on a big serving platter.