Cook the Noodles. Start by boiling the egg noodles according to the package instructions, then drain and set aside.
Prepare the Tuna Mixture. In a large mixing bowl, combine the cooked noodles, canned tuna (drained), peas, and chopped onion. Stir in the cream of mushroom soup and milk, then season with salt and pepper to taste. The mixture should be creamy and well combined.
Assemble the Casserole. Transfer the tuna mixture into a greased 9x13-inch baking dish. Spread it out evenly to ensure it bakes uniformly.
Add the Cheese and Chips. Top the casserole with shredded cheddar cheese, making sure it covers the surface. Then sprinkle a generous amount of chips over the cheese for a crispy topping.
Bake. Preheat your oven to 350°F. Place the casserole in the oven and bake for 20–25 minutes, or until the cheese is bubbly and the top is golden brown. You can also broil it for 1–2 minutes at the end for an extra crispy topping.
Serve and Enjoy. Let the casserole cool slightly before serving. It pairs wonderfully with a simple side salad or steamed vegetables.