Preheat the oven to 350°F (175°C). Grease a 9×13 inch casserole dish with butter or non-stick spray.
Cook the egg noodles according to package instructions until al dente. Drain and set aside.
In a large skillet, melt 2 tablespoons of butter over medium heat. Add the chopped onion and sliced mushrooms and sauté until the onions are translucent and mushrooms are softened, about 5-7 minutes.
Sprinkle the flour over the onion and mushroom mixture and stir well to combine. Cook for 1-2 minutes to remove the raw flour taste.
Gradually whisk in the milk and chicken broth, stirring constantly to avoid lumps. Continue cooking and stirring until the sauce thickens, about 5 minutes.
Season the sauce with salt, black pepper, and garlic powder. Remove from heat.
In a large mixing bowl, combine the cooked noodles, tuna, thawed peas, and cheddar cheese. Pour the mushroom sauce over the mixture and stir gently to combine everything evenly.
Transfer the mixture to the prepared casserole dish and spread it out evenly.
In a small bowl, mix the dry breadcrumbs with the melted butter until the crumbs are evenly coated. Sprinkle the breadcrumb mixture evenly over the casserole.
Bake uncovered in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let it rest for 5 minutes before serving.