Preheat your oven to 350°F. In a large pot, bring water and ½ teaspoon of salt to a rolling boil. Add the egg noodles and cook for about 9 minutes until al dente. Drain and transfer the noodles to a 9×13-inch baking dish.

Add the frozen peas, cream of mushroom soup, drained tuna, and diced onion to the baking dish. Sprinkle with 2 teaspoons of salt, 1½ teaspoons of garlic powder, and 2 teaspoons of black pepper. Pour in the evaporated milk and stir until everything is well combined.

Cover the dish tightly with foil and bake at 350°F for 30 minutes. While the casserole bakes, combine the breadcrumbs and softened butter in a small bowl. Mix until the crumbs are evenly coated.

After 30 minutes, remove the foil and sprinkle the buttery breadcrumbs over the top. Increase the oven temperature to 400°F and bake uncovered for an additional 10 minutes, or until the topping is golden and crisp.