Instructions:
- Preheat oven to 375°F (190°C). Cook egg noodles until slightly underdone (al dente), drain, and set aside. Lightly grease a 9x13 inch (23x33 cm) casserole dish.
- Melt 4 tablespoons of butter in a large saucepan over medium heat. Add diced onion and celery. Sauté gently until softened (about 5–7 minutes). Add garlic and cook for 1 minute until fragrant.
- Sprinkle the flour over the cooked vegetables and butter mixture. Whisk constantly for 2 minutes to cook out the raw flour taste, forming a roux.
- Gradually whisk in the warm milk, ensuring no lumps form. Increase heat slightly and continue whisking until the sauce thickens enough to coat the back of a spoon (about 5–8 minutes). Bring just to a gentle simmer, then remove from heat.
- Whisk in Dijon mustard, Worcestershire sauce, salt, and pepper. Stir in 1 cup of the grated cheddar until completely melted and smooth to create the Mornay sauce.
- Gently fold the cooked noodles, flaked tuna, and frozen peas (if using) into the cheese sauce. Taste and adjust seasoning if necessary.
- Pour the mixture evenly into the prepared baking dish. Sprinkle the reserved 1/2 cup of cheddar cheese over the top.
- In a small bowl, toss the Panko breadcrumbs with the 2 tablespoons of melted butter and dried parsley until evenly coated. Sprinkle this topping evenly over the cheese layer.
- Bake for 15–20 minutes, or until the casserole is bubbling around the edges and the topping is a deep golden brown.
- Let the casserole rest for 5–10 minutes before serving to allow the sauce to set up properly.