Heat the oven to 200C/180 fan/gas 6. Whisk together the vinegar and half the olive oil. Put the squash in a roasting tin and toss with the dressing, the chilli and seasoning. Squeeze the sausagemeat out of the skins, roll into small meatballs (you should have about 35). Arrange around the squash and roast for 35-40 mins, turning halfway, until the squash is tender and the meatballs are caramelised.
Meanwhile, melt the butter in a saucepan over a medium heat, until bubbling. Add the sage and allow to sizzle for a few minutes, or until crisp. Remove using a slotted spoon and set aside on a plate lined with kitchen paper. By this point, the butter should have browned. Stir in the flour and cook for a few seconds, then gradually pour in the milk, whisking continuously until you have a smooth sauce, about 7-10 mins. Add the nutmeg and some seasoning, stir in the parmesan until melted, then stir in the mustard.
Oil a medium-sized baking dish. Cook the pasta following pack instructions until al dente, then drain and run under cold running water. Heat the remaining oil in a pan over a medium heat and fry the mushrooms with a pinch of salt for a few minutes, until softened and caramelised. Add the garlic, then the greens, stirring until the greens wilt, then add to the white sauce along with the squash, pasta and half the fried sage, adding up to 50ml milk or water if it’s too thick.
Pour into the baking dish, then add the meatballs. Scatter over the remaining sage, the reserved squash seeds, and grate over the extra parmesan. Roast for 25-30 mins, until golden and bubbling. Allow to settle for 10 mins before serving.