Coarse salt and ground pepper
6 tablespoons butter
1 pound white mushrooms, trimmed and sliced 1/4 inch thick
1/2 cup all-purpose flour
3 cups milk
1 can (14.5 ounces) reduced-sodium chicken broth
3 cups grated Parmesan cheese
1/2 teaspoon dried thyme leaves
1 pound linguine, broken in half
1 rotisserie chicken, skin removed, meat shredded (about 4 cups)
1 package (10 ounces) frozen peas, thawed and drained
Preheat oven to 400. Bring a large pot of salted water to a boil (for pasta). In a large saucepan, melt 2 tablespoons butter over high heat. Add mushrooms, and season with salt and pepper. Cook, tossing frequently, until tender and browned, 8 to 10 minutes. Transfer to a bowl, and set aside.
Make sauce: In same saucepan, melt remaining 4 tablespoons butter over medium heat. Add flour; cook, whisking, about 1 minute. Whisking constantly, gradually add milk, & broth. Bring to a boil; reduce to a simmer, and add 2 cups Parmesan and thyme. Season with salt and pepper.
Cook pasta 2 minutes less than package instructions for al dente; drain and return to pot. Add sauce, chicken, peas, and mushrooms. Toss well to combine. Add to baking dish, sprinkle with remaining Parmesan. Bake until browned, about 30 minutes. Let stand 10 minutes before serving.