Prepare noodles according to package directions, making sure to liberally salt the pasta water. Leave pasta in the strainer and return pot to the stove.
In the pasta pot, saute onion and mushrooms in the butter (with generous pinch of salt & small pinch of pepper) over medium heat until onions soften (5-8 min), stirring occasionally.
Add the flour to the pot and stir mixture for about a minute, then add full can of evaporated milk and whisk for ~3 minutes or until mixture has thickened a bit. Add squash puree to pot and whisk to combine, then add cheeses and pinch of nutmeg and continue mix until sauce is smooth.
Return the noodles to the pot and gently fold into the sauce with a spatula or wooden spoon.
Notes
Serve with some more sprinkles of Parmesan cheese, alongside a green salad or any vegetable side dish.
Recipe by Eating Made Easy at https://eating-made-easy.com/quick-easy-mac-n-cheese/