Preheat the oven to 375°F (190°C) and lightly grease a 9×13-inch baking dish.

Cook the egg noodles according to package directions until al dente. Drain and set aside.

In a large skillet over medium heat, heat the olive oil or butter and sauté the diced onion for 3–4 minutes until soft. Add garlic and cook for 1 minute more.

Sprinkle in the flour and stir continuously for 1–2 minutes to form a roux.

Slowly pour in the warm milk, whisking constantly to prevent lumps. Cook until the sauce thickens, about 3–4 minutes.

Stir in the sour cream, mustard powder, salt, and pepper. Remove from heat.

In a large bowl, combine the cooked noodles, flaked tuna, peas, 1 1/2 cups of cheddar cheese, and the creamy sauce. Mix well.

Transfer the mixture into the prepared baking dish and spread evenly. Top with the remaining 1/2 cup of cheddar cheese.

In a small bowl, toss the panko breadcrumbs with olive oil or melted butter. Sprinkle evenly over the casserole.

Bake uncovered for 20–25 minutes or until bubbly and golden brown on top.

Garnish with chopped parsley before serving, if desired.