Heat 1 tablespoon of the oil in a large, nonstick frying pan over a medium-high heat, add the chicken and fry for 3–4 minutes or until browned, stirring frequently. Reduce the heat slightly, add the leeks and cook for a further 3–4 minutes or until the leeks are translucent.
Meanwhile, heat the passata in a saucepan and stir in one-third of the chopped herbs, the fennel seeds and the remaining oil. Season with salt and pepper to taste, then simmer gently for 2–3 minutes.
Remove the chicken from the heat and stir in the ricotta, the remaining chopped herbs, the lemon rind, nutmeg, paprika, spinach and sun-dried tomatoes, if using. Season to taste. Stand the cannelloni tubes upright and use a narrow spoon to fill them with the ricotta mixture.
Spoon half of the passata over the base of a large, shallow ovenproof dish. Arrange the filled pasta tubes, side by side over the tomato sauce, so that they all fit in one closely fitting layer. Pour the remaining passata over the top of the cannelloni, then sprinkle with the Parmesan.
Place in a preheated oven, 180°C (350°F), Gas Mark 4, for 35–40 minutes or until cooked through and bubbling. Serve hot with a crisp green salad.