Preheat the oven to 180°C/350°F/gas 4. Grease a 30cm x 25cm baking dish.
Cook the pasta in a large pan of boiling salted water, for slightly less time than given on the packet so it’s just undercooked. Drain and run under cold water to stop it from cooking further. Return to the saucepan and coat with a drizzle of olive oil.
Meanwhile, roughly chop the leeks. Put the butter in a frying pan and gently fry the leeks for 10 minutes or until they’re soft. Peel, mince and add the garlic and cook for 1 minute. Add the stock and wine, then cook for 8 to 10 minutes, or until almost all the liquid has evaporated. Add the peas, stir and cook for 30 seconds.
Dice the cooked chicken.
To the drained pasta, add the leek mixture, chicken, milk and two-thirds of the ricotta. Mix gently to combine and season with salt and pepper.
Spoon the pasta mixture into the baking dish. Top with the remaining ricotta, grate over the Parmesan and drizzle with olive oil, if you like. Bake in the oven for 25 minutes or until piping hot and starting to crisp up on top.