Rinse the lentils in a sieve, then transfer to a saucepan with the water. Bring to the boil, then simmer on a low heat, covered, for 18-20 minutes or until all the liquid has been absorbed. Transfer to a plate and set aside to cool.
Preheat the oven to 180°C fan / 200°C / 400°F and line a baking tray with paper.
In a large mixing bowl, combine the onion, garlic, parsley, raisins, 2 tablespoons of harissa paste, chickpea flour, lemon zest, and a pinch of salt. When the lentils have mostly cooled, add them to the bowl and mix well.
Shape the mixture into 12 evenly sized balls and place them on the tray. Bake for 25 minutes, turning halfway.
Meanwhile, combine the cucumber and tomatoes in a bowl with juice from half the lemon and a pinch of salt.
Chop most of the mint leaves, saving a few for garnish. Mix the chopped mint with the yoghurt, the remaining lemon juice, and a pinch of salt.
In a small bowl, stir together the remaining 2 tablespoons of harissa paste with 2 tablespoons of olive oil.
Remove the lentil meatballs from the oven and, while they’re still warm, gently reshape them into neat balls. Brush all over with the harissa oil, then return to the oven for 1 more minute. Don’t worry if the balls feel a little soft when they come out of the oven - they’ll firm up as they cool slightly.
To serve, spoon the mint yoghurt into bowls, top with the tomato-cucumber salad, lentil meatballs, and pickled red onion. Finish with the reserved mint leaves, freshly ground black pepper, and any leftover harissa oil. Serve with warm flatbread.