Cook the pasta in a large pan of salted boiling water for 9 minutes.
Meanwhile, place the squash and red onion into a shallow frying pan with the olive oil and butter.
Add the garlic and rosemary as well as the California walnuts.
Finally add the flour and California walnuts and fold together.
Season with a little salt and cook for 5 minutes.
Add a little of the pasta cooking water to the pan to create an emulsion sauce.
When the pasta is al dente, use a pair of tongs to lift out the pasta and place in the pan with the squash and walnuts.
Stir together and finish with the grated parmesan cheese.
Taste and season with salt and pepper if needed.
