Step 1: Cook the Noodles

• Boil the noodles: Bring a large pot of salted water to a boil. Cook the egg noodles according to the package instructions until al dente, then drain and set aside.

Step 2: Sauté the Vegetables

• Cook the onions and garlic: Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for about 5 minutes, until softened. Add the minced garlic and cook for an additional 1 minute.
• Sauté the mushrooms: Add the sliced mushrooms to the skillet and cook until tender and browned, about 6-8 minutes.

Step 3: Make the Creamy Sauce

• Create the roux: Sprinkle the flour over the sautéed vegetables and stir for 1-2 minutes to cook off the raw flour taste.
• Add milk and cream: Slowly whisk in the milk and heavy cream, continuing to whisk until the sauce thickens, about 3-4 minutes.
• Season the sauce: Stir in salt and pepper to taste, then fold in the drained tuna and frozen peas. Cook for an additional 2 minutes to warm everything through.

Step 4: Assemble the Casserole

• Mix the noodles and sauce: In a large bowl, combine the cooked noodles with the creamy tuna sauce. Stir in 1 ½ cups of shredded cheddar cheese.
• Transfer to a baking dish: Grease a 9×13-inch baking dish and pour the mixture into the dish, spreading it out evenly.

Step 5: Add the Topping

• Prepare the topping: In a small bowl, mix the panko breadcrumbs, melted butter, grated Parmesan cheese, and remaining ½ cup of cheddar cheese.
• Sprinkle on top: Evenly spread the breadcrumb topping over the casserole.

Step 6: Bake the Casserole

• Bake: Preheat the oven to 375°F (190°C). Bake the casserole for 25-30 minutes, or until the top is golden brown and the casserole is bubbling.

Step 7: Serve and Garnish

• Garnish and serve: Remove the casserole from the oven and let it cool for 5 minutes. Garnish with fresh parsley before serving.