Heat oven to 200C/fan 180C/gas 6. Put the butternut squash in a steamer over boiling water. Steam for around 15-20 mins or until tender. Drain and then blitz in a food processor until smooth.
Cook the pasta according to the pack instructions.
Heat the butter in a medium saucepan, add the leek and cook for 2 mins. Stir in the flour and cook for 1-2 mins more. Take the pan off the heat and gradually whisk in the milk. Return to the heat and bring to the boil, stirring all the time. Simmer for 5 mins. Stir in the peas and bring back to a simmer. Take the pan off the heat and stir in the butternut squash, then 125g cheese.
Stir the pasta into the sauce and transfer to an ovenproof dish. Sprinkle over the remaining cheese and the breadcrumbs. Bake for 20 mins or until golden and bubbling.
