Preheat the oven to 375°F with a rack in the center position.
Bring a large pot of salted water to a boil over high heat. Add the noodles and cook according to the package directions until al dente. Drain and set aside.
In a medium saucepan over medium heat, combine the soup, milk, and cheddar cheese. Cook, stirring frequently, until the cheese is melted, about 5 minutes. Stir in the salt, pepper, and garlic powder, if using.
Place the cooked noodles, tuna, soup mixture, and peas, if using, in a 9 x 13-inch baking dish. Stir until well combined.
In a small bowl, toss the panko breadcrumbs with the melted butter. Sprinkle evenly over the casserole.
Bake for 30 to 40 minutes, or until bubbling in the center and slightly crisped around the edges. Let rest for 5 minutes before serving.