Cook the pasta in a large pan of boiling lightly salted water according to the packet instructions, until al dente. Add all the peas and the leeks to the pasta for the last 5 minutes or so of cooking.
When just tender, drain well, reserving about 250ml of the cooking water. Tip the pasta and vegetables into a large baking dish. Add the cubed ham 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 pan 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 from the heat and mix in the mature cheese. Season to taste with salt and pepper.
Pour the sauce over the pasta, stirring gently with a fork. Poke the tomato halves into the mixture, then sprinkle with the Parmesan, oregano, basil and breadcrumbs. Bake for 20 minutes until bubbling and crispy on top. Serve hot.
Use any short pasta shape you'd like - penne, shells, macaroni. For a lower GI version, use wholewheat pasta, which has a low GI value.