Preheat the oven to 375 degrees and lightly grease a 9x9 baking dish.
Bring a large pot of water to a boil and cook the egg noodles according to package directions until just tender. Drain and set aside.
In a large mixing bowl, stir together the cream of mushroom soup, milk, salt, and black pepper until smooth.
Add the drained tuna, frozen peas, shredded cheddar cheese, and cooked noodles to the bowl. Fold gently until everything is evenly coated.
Transfer the mixture to the prepared baking dish and spread it into an even layer.
Sprinkle the crispy fried onions evenly over the top.
Bake for 20 to 25 minutes until the casserole is bubbling around the edges and the onion topping is golden.