Preheat your oven to 350°F.
Grease a 9-x-13-inch baking dish.
Cook the pasta according to package instructions until it’s al dente, then drain it.
Set it aside.
In a large skillet over medium-high heat, melt butter.
Add mushrooms and onions, cooking until they’re soft and lightly browned.
Stir in the flour, then slowly add chicken broth, whisking until smooth.
Add the milk and let it thicken.
Season with salt and pepper.
Combine the pasta, chicken, and sauce.
Transfer it all to the greased baking dish.
Sprinkle Parmesan on top and bake for 25 minutes until golden and bubbly.