Preheat the oven to 190 degrees C. Cook the pasta in a large saucepan of boiling water for 10–12 minutes or according to the packet instructions, until tender. Trim the broccoli into small florets and slice the stalks. Add all the broccoli and the leeks to the pasta for the last 5 minutes or so of cooking.
When just tender, drain well, reserving about 1 cup (250ml) of the cooking water. Tip the pasta and vegetables into a large ovenproof dish. Skin the chicken and cut into small chunks and add to the dish.
Blend a little of the milk with the cornflour to a paste in a jug. Heat the remaining milk and reserved cooking water in the same saucepan until on the point of boiling. Pour about a cupful onto the cornflour paste and stir well, then pour this into the simmering milky water and stir briskly with a wooden spoon until it thickens. Remove the pan from the heat and mix in the mature cheese. Season to taste.
Pour the sauce over the pasta, stirring gently with a fork. Nestle the tomato halves into the mixture, then sprinkle with the parmesan, oregano and breadcrumbs. Bake for 20 minutes until bubbling and crispy on top. Serve hot.