Creamy Chicken Tetrazzini

Name: Creamy Chicken Tetrazzini
Ingredients: Cooking spray; 1 medium yellow onion; 3 cloves garlic; 1 ounce Parmesan cheese, finely grated (scant 1/3 cup store-bought or 1/2 cup grated on the small holes of a box grater); 1 tablespoon olive oil; 1/2 cup panko breadcrumbs; 5 tablespoons unsalted butter, divided; 1 pound sliced cremini mushrooms; 3 teaspoons kosher salt, divided, plus more for the pasta water; 1/2 teaspoon freshly ground black pepper; 1/3 cup all-purpose flour; 3 cups whole milk; 1 cup heavy cream; 1 cup low-sodium chicken broth; 1/8 teaspoon ground nutmeg; 12 ounces cooked boneless chicken (about 3 cups); 1 pound dried linguine; 1 cup frozen peas; 1/2 teaspoon Italian seasoning