6 tbsp. butter
¾ lb. white, cremini, or stemmed shiitake mushrooms, sliced
2 large shallots or 1 medium onion, finely chopped
1 large stalk celery with leafy top, finely chopped
2 large cloves garlic, finely chopped
Salt and 1 tsp. finely ground white or black pepper
¼ cup flour
½ cup white wine or 1/3 cup dry sherry
1 cup chicken stock
1 cup whole milk
1 bag (12 oz.) wide egg noodles
1 cup (loosely packed) mixed fresh dill, chives, and flat-leaf parsley leaves, finely chopped
½ cup sour cream or crème fraîche
1 rounded tbsp. Dijon mustard
½ cup frozen peas
2 cans (5 to 6 oz. each) sustainable tuna, drained and flaked
1 ½ cups crushed potato chips (about 3 oz.)
1 cup grated Parmigiano-Reggiano (about 4 oz.)
¼ cup chopped drained cornichons
Bring a large pot of water to a boil for noodles.
Preheat the broiler. Or position a rack in the center of the oven and preheat to 450°.
In a large skillet or pot, melt the butter over medium-high heat. When it foams, add the mushrooms and cook, stirring occasionally, until browned, about 5 minutes. Reduce heat a bit. Add the shallots, celery, and garlic. Season with salt and 1 tsp. pepper. Cook, stirring often, until the vegetables soften, about 3 minutes. Add the flour and stir to coat the vegetables. Add the wine and cook until absorbed, about 30 seconds. Whisk in the stock and bring to a bubble. Add the milk and cook until the sauce thickens and reduces by about a third, about 3 minutes.
Meanwhile, salt the boiling water and add the noodles. Cook until the noodles are almost tender, about 5 minutes. Reserve 1/2 cup of the cooking water, then drain the noodles.
Add half the herbs, the sour cream, and Dijon to the sauce. Stir in the peas and cook until heated through, about 1 minute. Fold in the tuna. Add the noodles and toss until coated, adding the cooking water if needed to thin the sauce. Transfer to a 3-quart casserole dish.
In a large bowl, mix the potato chips and Parm. Scatter over the casserole. Bake until the top is browned and the cheese melts, about 2 minutes. Top with the cornichons and remaining herbs.