Preheat oven to 400°F. Spray 13x9-inch baking dish with no stick cooking spray; set aside. Cook noodles in salted water as directed on package. Rinse with cool water. Drain well and set aside.
Meanwhile, melt 4 tablespoons of the butter in large deep skillet on medium-high heat. Add panko, cook and stir just until lightly browned, about 5 minutes. Transfer to small bowl. Stir in parsley; set aside. Carefully wipe out pan.
Melt remaining 4 tablespoons butter in same skillet on medium-high heat. Add onion and bell pepper; cook and stir 3 minutes until softened. Stir in mushrooms and garlic powder; cook, stirring occasionally, until mushrooms are tender about 3 minutes. Sprinkle flour into skillet. Add salt and pepper. Cook and stir 1 minute. Gradually whisk in milk. Bring to gentle boil. Reduce heat and simmer 3 minutes or until thickened. Remove from heat. Gently stir in noodles, tuna and peas. Spoon into prepared baking dish. Sprinkle top evenly with panko mixture.
Bake 20 to 25 minutes or until top is lightly browned. Let stand 5 to 10 minutes before serving.
