Preheat oven to 375°F and grease a 9x13-inch baking dish. Cook egg noodles according to package directions until al dente, drain, and set aside.
In a large skillet, melt 3 tablespoons butter over medium heat. Add onion and mushrooms, cooking until softened, about 5 minutes.
Sprinkle flour over the vegetables and stir for 1 minute. Gradually whisk in milk and chicken broth, stirring constantly until the sauce thickens, about 3-4 minutes.
Stir in tuna, peas, 1 cup of cheddar cheese, garlic powder, salt, and pepper. Remove from heat.
Add cooked noodles to the sauce and toss to combine. Transfer mixture to the prepared baking dish.
Mix panko breadcrumbs with 2 tablespoons melted butter and remaining ½ cup cheese. Sprinkle evenly over the casserole.
Bake for 25-30 minutes until golden and bubbling. Let stand 5 minutes before serving.