Preheat your oven to 350 degrees Fahrenheit. This is an important step, as a preheated oven ensures even cooking and browning.
In a large bowl, combine the cooked noodles, drained tuna, and mushrooms. Stir gently to ensure everything is evenly mixed. The noodles should be slightly cooled so they don’t cook further when mixed with other ingredients.
In a separate bowl, whisk together the cream of mushroom soup, milk, salt, and pepper. This creamy mixture will envelop your noodle combination, binding all the flavors together.
Pour the soup mixture over the noodle mixture, stirring until everything is well coated. This step is crucial as it ensures that every bite is creamy and flavorful.
Transfer the entire mixture into a greased 2-quart baking dish. Spread it out evenly with a spatula, ensuring that there are no dry noodles peeking out.
Now, it’s time to prepare the topping. In a small bowl, combine the crushed saltines and melted butter. Mix well until the cracker crumbs are evenly coated.
Sprinkle the cracker mixture evenly over the top of the casserole. This will create a delightful crispy layer once baked.
Bake in the preheated oven for 35 to 45 minutes. Keep an eye on it during the last few minutes; you're looking for the casserole to be bubbling and the top to be a lovely golden brown.
Once done, remove from the oven and let it sit for a few minutes before serving. This will allow the flavors to meld together beautifully.
