Preheat the oven to 350°F (175°C).
Cook the egg noodles according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, sauté the chopped onion and sliced mushrooms until softened, about 5 minutes.
In a large mixing bowl, combine the condensed cream of mushroom soup, milk, salt, and black pepper. Stir until smooth.
Add the cooked noodles, drained tuna, sautéed onions and mushrooms, frozen peas, and shredded cheddar cheese to the soup mixture. Stir gently to combine.
Transfer the mixture to a 9×13 inch baking dish and spread evenly.
In a small bowl, mix the crushed buttery crackers with the melted butter until the crumbs are evenly coated.
Sprinkle the buttered cracker crumbs evenly over the casserole.
Bake uncovered for 35 to 40 minutes, or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let stand for 5 minutes before serving.