Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Cook the egg noodles in a large pot of salted boiling water according to package directions, until al dente. Drain well and set aside.

In a large skillet or Dutch oven, melt 1/4 cup unsalted butter over medium heat. Add the diced onion and sliced mushrooms and sauté for 5-7 minutes, until softened. Add the minced garlic and cook for another minute until fragrant. Remove the vegetables from the skillet and set aside.

In the same skillet, melt the remaining 1/4 cup butter (if needed, otherwise just use the residual butter in the pan). Sprinkle in the all-purpose flour and whisk constantly for 1-2 minutes to create a roux, cooking out the raw flour taste.

Gradually whisk in the warmed milk and broth, a little at a time, ensuring no lumps form. Continue to whisk and cook until the sauce thickens to your desired consistency, about 5-7 minutes.

Remove the skillet from the heat. Stir in the shredded cheddar cheese until melted and smooth. Season with 1/2 tsp salt, 1/4 tsp black pepper, dried thyme, and Dijon mustard (if using). Taste and adjust seasonings as needed.

Gently fold the sautéed onion and mushrooms, drained flaked tuna, and frozen peas into the creamy sauce. Add the cooked and drained egg noodles and stir until everything is well combined and coated.

Pour the noodle mixture into the prepared 9x13 inch baking dish, spreading it evenly.

In a small bowl, combine the Panko breadcrumbs, 2 tablespoons of melted butter, and grated Parmesan cheese (if using). Sprinkle this topping evenly over the casserole.

Bake for 25-30 minutes, or until the casserole is bubbly around the edges and the topping is golden brown and crispy.

Remove from oven and let rest for 5-10 minutes before serving. Garnish with fresh chopped parsley, if desired.