Follow these steps carefully to achieve a perfectly creamy, cheesy, and delicious Tuna and Corn Casserole.

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). Grease a 9×13 inch (or similar 2.5-3 quart) baking dish lightly with butter or cooking spray.
2. Cook the Pasta: Bring a large pot of salted water to a rolling boil. Add the egg noodles (or your chosen pasta) and cook according to package directions until al dente – usually about 6-8 minutes. It’s important not to overcook the pasta, as it will continue to cook in the oven. Drain the pasta well and set aside.
3. Sauté Aromatics: While the pasta is cooking, melt the 2 tablespoons of butter in a large skillet or Dutch oven over medium heat. Add the chopped onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
4. Make the Roux: Sprinkle the all-purpose flour over the cooked onions and garlic. Stir continuously for 1-2 minutes to cook out the raw flour taste. This mixture (butter, aromatics, and flour) is called a roux, and it will thicken your sauce.
5. Create the Creamy Sauce: Gradually whisk in the milk, a little at a time, ensuring each addition is fully incorporated before adding more to prevent lumps. Once all the milk is added, whisk in the chicken or vegetable broth. Continue to cook, whisking frequently, until the sauce comes to a gentle simmer and begins to thicken, about 5-8 minutes.
6. Season and Add  Cheese: Once the sauce has thickened, remove it from the heat. Stir in the salt, black pepper, and dried thyme or Italian seasoning (if using). Add 8 ounces of the shredded cheddar cheese and stir until the cheese is completely melted and the sauce is smooth and luscious. Taste and adjust seasonings if necessary.
7. Combine Ingredients: To the skillet with the cheese sauce, add the drained cooked pasta, the drained and flaked tuna, the drained corn, and the frozen peas (if using). Gently stir everything together until well combined and all the pasta and other ingredients are coated in the creamy sauce.
8. Assemble the Casserole: Pour the tuna and corn mixture evenly into the prepared baking dish.
9. Add Topping and Bake: Sprinkle the remaining 1/2 cup of shredded cheddar cheese evenly over the top of the casserole. Then, sprinkle the crushed potato chips, panko breadcrumbs, or crushed butter crackers uniformly over the cheese.
10. Bake to Perfection: Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the casserole is hot and bubbly around the edges, and the topping is golden brown and crispy. If the topping starts to brown too quickly, you can loosely tent the dish with aluminum foil for the remainder of the baking time.
11. Rest Before Serving: Once baked, carefully remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the sauce to set slightly and makes it easier to serve, plus it prevents burning your mouth!