Heat the oil in a deep frying pan with a lid over a medium heat. Fry the chicken for 2-3 mins on each side until golden. Remove from the pan. Add most of the onion with a pinch of salt. Fry for 10 mins until soft, then stir in the garlic and harissa. Fry for 2 mins until fragrant. Tip in the tomatoes, half-fill the can with water, swill a few times, then add to the pan. Stir in the lentils with their water. Bring to a simmer, nestle the chicken back into the pan, lower the heat to low-medium, add the lid so it’s half covered, and simmer for 35-40 mins until the chicken is tender. Shred the meat into the sauce with two forks. Simmer uncovered for a few minutes to thicken the sauce if you need to. Season to taste.