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Heat the oven to 375 degrees and grease a 9-by-13 or other 3-quart casserole dish with butter.
Bring a large pot of salted water to a boil. Add the pasta and cook according to the package instructions until just al dente. (Do not overcook.) Drain and set aside.
Meanwhile, in a medium Dutch oven or other heavy pot, melt the butter over medium heat. Add the celery and onion, season with salt and cook over medium, stirring occasionally, until the onions are translucent, 6 to 8 minutes. Add the mushrooms and cook, stirring occasionally, until they are tender, about 5 minutes. Add the capers and garlic and cook until the garlic is fragrant, about 1 minute.
Sprinkle the flour over the vegetables and cook, stirring constantly, until no streaks of flour remain, about 1 minute, reducing the heat as necessary to keep from scorching. Add the stock, half-and-half, sherry, 2 teaspoons salt and 1/2 teaspoon pepper and bring to a boil.
Reduce the heat to low and simmer, stirring occasionally and scraping the bottom of the pan, until the sauce has thickened slightly, 5 to 7 minutes. Off the heat, stir in the tuna, peas and parsley, if using; season to taste.
Add the cooked noodles and toss well. Transfer the mixture to the prepared pan and smooth into an even layer. Sprinkle the crushed potato chips on top and bake, uncovered, until hot and bubbling, about 25 minutes. Garnish with additional parsley, if using, and serve hot.