1. Heat the oil in a large frying pan over a medium heat. Fry the leeks for 7-8 mins or until defrosted, softened and the water has evaporated, then push to the side of the pan and add the chicken. Cook for 6-7 mins or until the meat is browned and cooked through.
2. Meanwhile, cook the pasta to pack instructions. Drain, reserving 350ml pasta water, and set aside.
3. Stir the garlic into the chicken and cook for 1 min. Add 100ml pasta water, reduce the heat to low and stir in the butter and the cooked pasta, tossing to coat.
4. Add another 100ml pasta water and scatter with half the cheese and add a good grinding of black pepper. Remove from the heat and set aside for 1 min. Add another 100ml or so of pasta water and more of the cheese, saving a little to serve, to make a sauce with the consistency of single cream. Stir to combine, then divide between 4 bowls and top with the remaining Parmesan, the parsley and chilli, if using.
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