Preheat the oven to 350°F (175°C).
Cook the egg noodles in a large pot of boiling salted water according to package instructions until al dente, about 7 minutes. Drain and set aside.
In a large skillet, melt 2 tablespoons of the butter over medium heat. Add the chopped onion and sliced mushrooms and sauté until softened and lightly browned, about 5 minutes.
In a large mixing bowl, combine the condensed cream of mushroom soup, sour cream, milk, garlic powder, salt, and black pepper. Stir until smooth.
Add the drained tuna, thawed peas, sautéed onions and mushrooms, shredded cheddar cheese, and cooked egg noodles to the bowl. Gently fold everything together until well combined.
Transfer the mixture to a greased 9×13-inch baking dish and spread evenly.
In a small bowl, melt the remaining 1 tablespoon of butter. Add the plain dry breadcrumbs and toss to coat evenly.
Sprinkle the buttered breadcrumbs evenly over the casserole.
Bake uncovered in the preheated oven for 30 minutes or until the casserole is bubbly and the breadcrumb topping is golden brown.
Remove from the oven and let it rest for 5 minutes before serving.