Preheat the oven to 350 degrees. Prepare two 9x13 casserole dishes with nonstick cooking spray.
Cook the linguine according to package directions, approximately 8 minutes in boiling water. Drain well.
In a stock pot or dutch oven, melt the butter over medium heat. Add the onion and cook for 3 to 5 minutes or until it begins to soften. Add the mushrooms and continue cooking for 3 to 5 additional minutes or until the mushrooms are softened.
Add the flour and seasonings, stirring to combine. Cook for about 1 minute as you stir. Slowly add the milk, stirring constantly. Allow the sauce to thicken until it coats the back of a spoon.
In a large bowl, toss together the pasta, sauce, peas, cheese, and chicken, stirring until evenly blended. Divide between the two casserole dishes.
Top the casseroles with the bread crumbs.
Bake for 30 to 40 minutes or until the edges bubble and begin to brown slightly.