Melt 2 Tablespoons of butter in a large pot over medium heat.
Once hot and melted, add the onions and cook until golden, about 4 minutes.
Add the sliced mushrooms and cook until your desired color and texture. (We did 4 minutes.)
Remove the onions and mushrooms to a small bowl.
Place the remaining butter in the pot.
Meanwhile, prepare the pasta according to package directions.
Whisk the flour into the hot, melted butter to form a roux, then whisk in the milk. Season with salt, pepper and thyme and cook for 8 minutes until thickened.
Stir in the chicken broth, pasta, green peas and cooked chicken and cook until chicken is warmed through.
Set oven to broil.
Pour the pasta mixture into an 8" x 8" casserole dish and sprinkle the grated Parmesan cheese overtop.
Place the casserole dish in the oven and broil until cheese is golden and melted.
Notes
This Recipe Can be Doubled: This is a great recipe to bring to potlucks or family reunions. Just increase the ingredients proportionately and split it into two casserole dishes before broiling.
Vegetables: If peas or mushrooms are not a favorite or your picky eaters won't touch them, swap for other vegetables that you like better. You can use veggies such as red peppers, spinach, broccoli, or zucchini.
Easy Turkey Tetrazzini: You can use turkey in place of chicken. This is an easy to transform leftover turkey into something delicious.
Cream of Chicken: In place of chicken broth, you can use cream of chicken soup or cream of mushroom soup.