1.Add to large pot and place pot on stove over medium-high heat. Bring water to boil, then add 1/2 tablespoon salt. Stir to incorporate. Spray a 3-quart baking dish with cooking spray.
2.When salted water begins to boil, add noodles and stir to incorporate. Boil noodles for 3 minutes then immediately pour noodles into colander. Drain noodles very well, then transfer noodles to baking dish, set aside.
3.Preheat oven to 350°F. Add 5 tablespoons butter to medium saucepan and place pan on stovetop over low heat. Wait until butter is almost completely melted, stirring butter occasionally. Add flour to saucepan, vigorously whisk butter and flour together until fully incorporated into smooth mixture. Cook flour mixture over low heat approximately 1 minute, stirring constantly, until mixture turns golden and begins to bubble.
4.Remove saucepan from heat. Slowly add milk in small increments, whisking constantly to incorporate each addition. When all milk has been added, return saucepan to heat and increase heat to medium. Simmer mixture over medium heat, whisking constantly, until mixture has thickened and begins to bubble.
5.Add 1/2 teaspoon salt, black pepper, garlic powder, and dried parsley, whisking gently to incorporate spices. Remove saucepan from heat. Add 1 cup cheddar cheese in small increments, whisking constantly, until all cheese has been incorporated and melted.
6.Immediately pour cheese sauce over egg noodles in baking dish, then add tuna and frozen green peas. Gently stir ingredients together until well blended and fully covered in cheese sauce. Spread mixture out evenly to cover entire bottom of baking dish, making sure to reach all sides.
7.Place casserole dish in oven. Bake for 15 minutes. Meanwhile, place 1 tablespoon butter in a small dish and microwave to melt. Add 2 tablespoons breadcrumbs and 1 large pinch of salt. Stir ingredients together until well blended.
8.After 15 minutes, remove casserole dish from oven. Sprinkle 1/2 cup cheddar cheese across top of dish, covering entire casserole evenly, then sprinkle breadcrumbs evenly over layer of cheese.
9.Return baking dish to oven. Bake casserole 5 minutes more or until cheese layer is melted and bubbly, then carefully remove dish from oven and set aside. Let casserole rest 3 to 5 minutes. Garnish casserole with chopped fresh parsley and serve warm.