Heat the oven to 375°F 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 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, paprika, and Herbes de Provence, and cook, stirring occasionally, until the onions are translucent, 6 to 8 minutes. Add the mushrooms and cook, stirring occasionally, until 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 heat as needed to keep from scorching. Add the stock, half-and-half, sherry, 2 teaspoons salt, and ½ teaspoon pepper, and bring to a boil.
Reduce 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, cayenne pepper, and chives. Serve hot.