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Preheat oven to 375°F. Coat a 9-by-13-inch pan (or similar-size 3-quart baking dish) with cooking spray.
Bring a large pot of water to a boil and cook noodles according to package instructions. Drain and set aside. Dry the pot.
Heat butter and 1 tablespoon oil in the pot over medium heat until the butter is melted. Add onion and celery; cook, stirring occasionally, until soft, about 5 minutes. Add mushrooms and cook, stirring occasionally, until their liquid evaporates, 3 to 5 minutes. Stir in sherry, soy sauce, salt and pepper and cook until almost all the liquid has evaporated, 2 to 4 minutes more. Sprinkle flour over the vegetables, stir to coat and cook for 30 seconds.
Increase heat to high, add 1/2 cup broth and stir until starting to boil. Add the remaining 1 1/2 cups broth, milk and mustard; bring to a boil and cook, stirring often, until the sauce thickens, about 2 minutes. Add tuna, spinach and the reserved noodles; cook, gently stirring often, until heated through, about 3 minutes. Transfer to the prepared baking dish.
Combine breadcrumbs and cheese in a small bowl. Stir in the remaining 1 tablespoon oil and sprinkle over the casserole. Bake until the sauce is bubbling and the topping is golden brown, 20 to 25 minutes.