Gather and prepare all ingredients.

Finely dice the onion and add it to a skillet with 1 Tbsp butter. Sauté the onion over medium heat until softened, 3-5 minutes.

While the onion is cooking, slice the mushrooms. Add them to the skillet with the onion and ¼ tsp each of salt and pepper. Continue sautéing over medium heat until the mushrooms have softened, for about 5 minutes. Remove the mushrooms and onions from the skillet and set aside.

Add the flour and an additional 2 Tbsp butter to the skillet. Cook and whisk over medium heat as the butter melts. Continue whisking as the butter and flour bubbles and foams for about one minute.

Add the frozen peas and the cooked mushrooms and onions back to the sauce. Stir to combine and season with the remaining salt. Set the sauce aside.

Begin to preheat the oven to 350ºF. Bring a large pot of water to a boil for the noodles. Once boiling, add the egg noodles and boil for about 7 minutes, or just until tender. Drain the noodles in a colander.

While the noodles are boiling, prepare the breadcrumb topping. Melt 1 Tbsp butter, then stir in the breadcrumbs and Parmesan.

Combine the creamy mushroom sauce, cooked and drained noodles, and drained tuna.

Transfer the casserole mixture to a 9x9-inch (3-quart) casserole dish, then top with the breadcrumb topping.

Bake the casserole for about 25 minutes or until heated through and golden brown on top. Serve hot and enjoy!