• Preheat oven to 350º Fahrenheit.

For the Noodles

• Cook according to package directions in heavily salted water, for shortest amount of time recommended.
• Drain and transfer to baking dish. Set aside.

For the Sauce

• Melt 5 tablespoons butter in heavy medium saucepan over low heat. Add flour. Stir until smooth.
  
• Cook 1 minute, stirring constantly until mixture bubbles and turns golden. Slowly add milk, whisking constantly. Cook over medium heat, whisking constantly, until thickened and bubbly.
  
• Stir in salt, pepper, garlic powder, and dried parsley. Remove from heat. Add 1 cup shredded cheese, stirring until melted
  

For the Casserole

• Pour sauce over drained noodles in baking dish. Add tuna and green peas and stir to combine. Bake in oven 15 minutes.
  
• While casserole is baking, melt remaining tablespoon butter in small bowl. Stir in breadcrumbs and large pinch of salt.
• After casserole bakes 15 minutes, sprinkle remaining ½ cup shredded cheese on top, then sprinkle buttery breadcrumbs over cheese.
• Return to oven and bake 5 minutes or until bubbly. Garnish with chopped parsley, if desired.