Step 1: Prep and Cook the Noodles
Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish (or a similar 3-quart casserole dish) with butter or cooking spray.

Bring a large pot of heavily salted water to a rolling boil. Add the egg noodles and cook according to package directions, but undercook them by about 2 minutes. They will finish cooking in the oven. This is a crucial step to prevent mushy noodles. Once cooked, drain the noodles well and set them aside.

Step 2: Sauté the Aromatics
In a large Dutch oven or a heavy-bottomed pot, melt 4 tablespoons of the unsalted butter over medium heat. Add the finely chopped yellow onion and cook, stirring occasionally, until it has softened and become translucent, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.

Step 3: Create the Creamy Cheese Sauce (Béchamel)
Sprinkle the all-purpose flour over the cooked onions and garlic. Whisk constantly for about 1-2 minutes to cook out the raw flour taste. This mixture is called a “roux,” and it will thicken our sauce.

Slowly, and in a steady stream, pour in the warmed milk while whisking vigorously to prevent any lumps from forming. Once all the milk is incorporated, whisk in the chicken or vegetable broth. Continue to cook over medium heat, stirring frequently, until the sauce begins to bubble and thicken, about 5-8 minutes. It should be thick enough to coat the back of a spoon.

Step 4: Add the Cheese and Seasonings
Remove the pot from the heat. Add 2 cups of the freshly grated sharp cheddar cheese, the Dijon mustard, salt, pepper, and optional hot sauce. Stir until the cheese is completely melted and the sauce is smooth and velvety. Be sure to taste the sauce at this point and adjust the seasoning if necessary. It should be well-seasoned, as the noodles and tuna will absorb a lot of the flavor.

Step 5: Assemble the Casserole
To the pot with the cheese sauce, add the drained, cooked egg noodles, the well-drained and flaked tuna, and the frozen peas. Gently fold everything together until all the ingredients are evenly coated in the glorious cheese sauce.

Pour the entire mixture into your prepared 9×13 inch baking dish and spread it into an even layer.

Step 6: Prepare the Topping and Bake
In a small bowl, melt the remaining 2 tablespoons of butter in the microwave. Add the Panko breadcrumbs to the melted butter and toss to combine until the crumbs are evenly moistened.

Sprinkle the remaining 1 cup of shredded cheddar cheese over the top of the casserole. Follow this with an even layer of the buttery Panko breadcrumbs.

Bake, uncovered, at 375°F (190°C) for 25-30 minutes, or until the sauce is bubbly around the edges and the Panko topping is golden brown and crispy.

Step 7: Rest and Serve
Remove the casserole from the oven and let it rest for at least 10-15 minutes before serving. This allows the sauce to set up slightly, making it easier to serve. Garnish with fresh chopped parsley for a pop of color and freshness, if desired