If desired, substitute one 10¾-ounce can condensed cream of mushroom soup mixed with ¾ cup milk for the butter/mushroom/onion/flour/milk mixture.
Preheat the oven to 375°F. Grease an 8-inch square baking dish.
Cook in a large pot of boiling salted water:
Drain well. Melt in a medium saucepan over medium heat:
Add and cook, stirring occasionally, until the vegetables are just tender, about 5 minutes:
Stir in and cook for 1 minute:
Gradually whisk in:
Bring the sauce to a boil, whisking, then turn the heat down and simmer for 10 minutes. Remove from the heat and add, whisking until hot:
Add the drained pasta to the sauce along with:
Stir together well. Pour the mixture into the prepared baking dish. Mix together and sprinkle over the top:
Bake until browned on top, 25 to 35 minutes.