Combine 1 teaspoons fine sea salt, 1/2 teaspoon black pepper, and 1/2 teaspoon garlic powder in a small bowl. Season 2 boneless skinless chicken breasts on all sides with the seasoning mixture. Set aside.
Heat 2 tablespoons olive oil in a large oven safe skillet over medium-high heat.
When the oil is warm, place the chicken in the pan and cook for 4-5 minutes on each side, cooking until the chicken is just cooked through (165°F). Remove the chicken from the pan and set aside to cool. When cooled, shred the chicken using 2 forks.
Reduce the heat to medium-low and add the remaining 2 tablespoons olive oil to the pan if needed. Sauté the chopped 1/2 yellow onion in the pan and toss it frequently until soft and fragrant, approximately 2-3 minutes. Add the sliced 2 cups baby bella mushrooms and cook for another 3 minutes. Finally, add the 1 cup frozen green peas along with the remaining 1 teaspoons fine sea salt and sauté for another 2 minutes. Vegetables should be soft. Remove the vegetables from the pan and set aside.
Reduce heat to low and add the 4 tablespoons unsalted butter to the pan. When the butter is fully melted and starts to bubble, add the 4 tablespoons flour and mix into a paste. Slowly pour in the 1/2 cup heavy cream as you stir the mixture with a wooden spoon.
Keep stirring as you pour in the 1 1/2 cups milk in small increments. You should be left with a smooth creamy mixture. Pour in 1 cups Parmesan cheeseand stir until just melted.
Add the cooked 12 ounces broad egg noodles, shredded chicken and sautéed vegetables into the pan with the cheese sauce and toss to combine. Top the mixture with the remaining cup of 1 cups Parmesan cheese and the 1/2 cup bread crumbs.
Bake in the oven for 20 minutes at 375°F. Top with fresh parsley and serve warm.
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Notes
Storage: You can store the leftovers in an airtight container or covered with plastic wrap in the refrigerator for up to four days. For best results, reheat this chicken tetrazzini in an oven safe container in the oven over low heat until it is warmed through.Freezing: Chicken tetrazzini is a freezer friendly casserole recipe, so why not make two when you are at it? Simply cover with foil and freeze for up to 3 months. To reheat, place in the oven at 350°F while still frozen and covered, bake for 45 minutes then remove foil and continue to cook for other 15-20 minutes or until the center has reached an internal temperature of 165°F.