Cook the noodles: Bring a large pot of salted water to a boil. Cook the egg noodles until just al dente according to package directions. Drain well and set aside.
Prepare the vegetables: Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish. Melt the butter in a large skillet over medium heat. Add the onion and mushrooms and cook for 4–5 minutes, until softened. Add the garlic and cook for 30 seconds.
Make the sauce: Sprinkle the flour over the vegetables and stir for 1 minute. Gradually whisk in the chicken broth and milk. Cook for 3–5 minutes, stirring frequently, until the sauce thickens.
Add the creamy ingredients: Reduce the heat to low. Stir in the sour cream, salt, black pepper, garlic powder, and paprika.
Add cheese and peas: Remove from the heat and stir in 1 cup of Cheddar until melted. Stir in the frozen peas.
Combine: Add the cooked noodles and drained tuna to the skillet. Gently fold until everything is evenly coated with the sauce.
Transfer to the dish: Spread the mixture evenly in the prepared baking dish. Sprinkle the remaining ½ cup Cheddar over the top.
Prepare the topping: Combine the panko breadcrumbs, melted butter, and Parmesan cheese in a small bowl. Sprinkle evenly over the casserole.
Bake: Bake uncovered for 20–25 minutes, until hot and bubbling and the topping is golden brown.
Rest and serve: Let the casserole rest for 5–10 minutes before serving.