Preheat oven to 375°F; grease a 9x13-inch dish. Cook noodles according to package until al dente; drain and set aside.
Melt 3 tablespoons butter in a large skillet over medium; cook onion and mushrooms about 5 minutes until softened.
Sprinkle flour over vegetables; stir 1 minute. Gradually whisk in milk and broth, stirring constantly 3-4 minutes until sauce thickens.
Stir in tuna, peas, 1 cup cheddar, garlic powder, salt, and pepper; remove from heat.
Toss noodles in sauce; transfer to dish.
Mix panko with 2 tablespoons melted butter and remaining ½ cup cheese; sprinkle evenly over casserole.
Bake 25-30 minutes until golden and bubbling; stand 5 minutes before serving.