Heat half the oil in a frying pan, season the thighs and brown really well on both sides. Remove to a plate. Tip a mug of water into the pan and simmer, scraping up all the browned bits. Tip this liquid into a measuring jug for later.
Put the remaining oil, the vegetables and coriander stalks in a big saucepan. Gently cook until the veg is softened – about 5 mins. Stir in the spices, turn up the heat, and cook for a few mins. Stir in the harissa, followed by the tomatoes, the chicken, stock cubes and lentils. Top up your jug of chickeny juices to 500ml with water, then add this, too. Bring to a simmer, cover and cook for 30 mins.
Lift the chicken from the soup and shred finely using a couple of forks. Return to the soup with the lemon zest and juice, sugar, and season to taste. Ladle into bowls with a dollop of yogurt and the coriander leaves, and serve with toast, spread with houmous and drizzled with oil.