(While you are making this sauce, prepare the vegetables and pasta you will dress with the sauce. It's a good back up plan to save a cup of the pasta water in case you need to loosen the sauce at the end.)
In a small, heavy saucepan over medium heat, melt the butter or margarine.
Add the flour to the butter, whisking to break up lumps. Allow the flour and butter to cook until the flour just starts to color -- about 2 minutes.
Add the milk, whisking to avoid lumps. Turn the heat to medium-low and allow the sauce to come to a slow simmer. Stir frequently and keep the heat relatively low to prevent scorching on the bottom.
Once the sauce has thickened to the consistency of heavy cream, whisk in the squash and heat through, stirring frequently.
Add salt, pepper, and other herb or spice to taste. Freshly grated nutmeg, dried basil, or rubbed sage are likely candidates.
Just before adding the sauce to the pasta (and vegetables), remove from the heat and whisk in the parmesan until smooth. Taste and do a final adjustment of the spices if necessary.
Toss drained pasta and veggies in the sauce. If the sauce is too thick, add some pasta water and toss again. Serve immediately.