Preheat the oven to 180°C.
Add the vegetable oil to a fry pan over medium heat. Fry the onion and leeks for about 5 minutes, or until everything has softened.
While the onion and leeks are cooking, boil a saucepan full of water and add the potatoes. Cook until tender (about 15 minutes).
Return to the pan. Add the minced chicken, carrots and courgette. Cook for an additional 6 minutes.
Add the passata, hot stock and herbs to the pan; put the lid on, reduce the heat, and simmer for 5 more minutes.
When your potatoes have finished boiling, mash them with the butter and milk and set aside.
Oil a medium sized oven dish and start by spreading the chicken and vegetable mixture along the base of the oven dish.
Spread the mash over the top and dot with butter.
Place in the oven for about 20 minutes or until the top is golden.
Serve with peas or steamed vegetables.