Sauté the vegetables. Place a large frying pan over medium heat. Add the drained mushrooms, minced garlic, diced yellow onion, and diced green bell pepper. Sauté until the vegetables become tender and aromatic. Remove the mixture from the heat and reserve it for later.

Cook the noodles. Bring a large pot of salted water to a rolling boil. Add the wide yolk-free egg noodles and prepare them according to the package instructions until they reach an al dente texture. Drain well and set aside.

Prepare the cream cheese sauce. Melt the butter in a large frying pan or saucepan over medium-low heat. Whisk in the flour and continue stirring constantly for 1–2 minutes to form a smooth roux.

Thicken the sauce. Slowly pour the milk into the roux while whisking continuously to prevent lumps. Continue cooking until the mixture develops a thick, smooth consistency. Add the cream cheese and stir until it has completely melted and blended into the sauce.

Combine the casserole filling. Add the cooked noodles to the prepared sauce. Stir in the undrained tuna, drained peas, and reserved sautéed vegetables. Season with 1 teaspoon salt and 1 teaspoon black pepper. Fold the ingredients together gently until evenly incorporated.

Assemble the casserole. Lightly grease a large 9 x 12-inch baking dish. Transfer the creamy tuna and noodle mixture to the dish and spread it into an even layer.

Add the topping. Scatter the grated sharp cheddar evenly across the casserole. Follow with the crispy fried onions, then finish with an even layer of grated Parmesan cheese.

Bake. Place the casserole in an oven preheated to 375°F (190°C). Bake for 20 minutes, or until the filling is bubbling and the surface has developed a golden-brown finish.

Rest before serving. Allow the baked casserole to stand for 5–10 minutes before slicing and serving.