Bring a large saucepan with lightly salted water to the boil and cook pasta according to instructions on the packet until al dente.
Meanwhile heat oil in a frying pan and cook and stir onion until soft. Add mince and fry, stirring frequently, until browned. Season with rosemary, sage and salt and pepper.
Melt butter in a small saucepan and add flour. Brown for a minute, stirring constantly and take off the heat. Pour in the milk slowly and whisk. Return to the heat, continue whisking and cook for a few minutes, until thickened and smooth. Season with salt, pepper and nutmeg to taste.
Drain pasta, add the cooked squash and place into a serving bowl. Add the mince and sauce and toss carefully.