Step 1: Preheat the Oven and Prepare the Baking Dish

• Preheat your oven to 375°F (190°C). This temperature allows the casserole to cook through evenly and the cheese to melt beautifully without burning the topping.
• Grease a 9×13 inch baking dish with butter, cooking spray, or olive oil. This prevents the casserole from sticking to the dish and makes serving easier. You can also use a slightly smaller or larger dish, adjusting the baking time accordingly. A smaller dish will result in a thicker casserole, while a larger dish will create a thinner layer.

Step 2: Prepare the Tuna Mixture

• In a large bowl, combine the drained tuna, mayonnaise, diced celery, and diced onion (or shallot). Use a fork to gently flake the tuna and mix everything together until well combined.
• Season the tuna mixture with salt and pepper. Taste and adjust the seasonings as needed. Add any optional seasonings like garlic powder, onion powder, dried dill, paprika, or hot sauce to your liking. Remember to season generously, as the flavors will mellow out during baking.
• Stir in 1 cup of the shredded cheddar cheese. This cheese will melt into the tuna mixture, adding creaminess and flavor throughout the casserole. Reserve the remaining 1/2 cup of cheese for topping.

Step 3: Prepare the Cream Sauce

• In a separate bowl, whisk together the condensed cream of mushroom soup and milk (or chicken broth). Whisk until smooth and creamy. This thins out the soup slightly, creating a pourable sauce that will coat the casserole ingredients.
• If you are using homemade cream sauce instead of condensed soup, prepare it according to your recipe. Ensure it is smooth and creamy before proceeding.

Step 4: Assemble the Casserole

• Spread the chosen casserole base (cubed bread, cooked pasta, or cooked diced potatoes) evenly in the prepared baking dish. Create a single layer of your base, ensuring it covers the bottom of the dish. If using bread cubes, you may need to gently press them down to create a more even layer.
• Pour the cream sauce evenly over the base layer. Make sure the sauce distributes evenly and coats the base ingredients.
• Spoon the tuna mixture evenly over the cream sauce layer. Spread it out to create a uniform layer over the sauce.
• Sprinkle the remaining 1/2 cup of shredded cheddar cheese evenly over the tuna mixture. This cheese will melt and create a golden, cheesy topping.
• If using breadcrumbs for the topping, toss them with the melted butter or olive oil in a small bowl. This will help them brown and crisp up in the oven.
• Sprinkle the breadcrumbs or crushed potato chips evenly over the cheese layer. Distribute the topping evenly for maximum crunch in every bite.

Step 5: Bake the Casserole

• Bake in the preheated oven for 25-30 minutes, or until the casserole is heated through, bubbly around the edges, and the topping is golden brown. The cheese should be melted and gooey, and the casserole should be heated all the way through.
• If the topping is browning too quickly, you can loosely tent the casserole with aluminum foil for the last 10-15 minutes of baking. This will prevent the topping from burning while allowing the casserole to continue cooking through.

Step 6: Broil for Extra Browning (Optional)

• For an extra golden brown and crispy topping, you can broil the casserole for the last 1-2 minutes of baking. Keep a close eye on it while broiling, as it can burn quickly. Broil just until the topping is nicely browned and bubbly.

Step 7: Let it Rest and Serve

• Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set slightly and makes it easier to serve. It also prevents burning your mouth on extremely hot cheese.

Step 8: Enjoy!

• Serve the Tuna Melt Casserole hot and enjoy its cheesy, comforting goodness! Pair it with your favorite side dishes (see “How to Serve” section below) for a complete and satisfying meal.