1 cup finely grated sharp white cheddar, plus more for topping (about 4 ounce total)
2 tablespoons roughly chopped yellow celery leaves, plus more for topping
2 tablespoons roughly chopped parsley leaves, plus more for topping
1 tablespoon minced chives, plus more for topping
1 tablespoon dried thyme
2 tablespoons freshly squeezed lemon juice
1 cup crushed potato chips
Directions
Make the celery root purée: in a large saucepan over medium heat, heat the oil. Add the onion, carrot and celery and cook, stirring often, until the onions are translucent, about 12 minutes. Add the celery root, milk, water and 1 teaspoon salt. Bring to a gentle boil, then simmer until the celery root is soft, about 7 minutes. Strain, reserving the cooking liquid. Transfer the strained vegetables to a food processor, add the butter, and process until smooth. Add reserved cooking liquid, about 3 cups, until the mixture has the consistency of a very thick soup. Season to taste with additional salt and pepper.
Prepare the noodles: Bring a large pot of water to a boil over high heat. Add enough salt to the water so that it tastes salty like the ocean. Cook the noodles until al dente, about 6 minutes. Strain the noodles and set aside. Rinse the pot out.
Assemble the casserole: in the clean pot, heat the olive oil over medium-low heat. Add the celery root and cook, stirring often, until just tender (you want some bite left in the vegetables so the casserole has texture), about 10 minutes. Add the celery and cook until just tender and bright green, about 6 minutes more. Season to taste with salt and pepper. Add cooked egg noodles and enough celery root purée to generously coat the noodles, about 4 cups. (If needed, set the pot over medium-low heat to heat through, adding a little cold water as needed to loosen the sauce.) Add the butter, trout and cheese and mix gently to keep the fish in large chunks. Stir in the celery leaves, parsley, chives, thyme and lemon juice. Season to taste with salt and pepper.
Transfer the mixture to a casserole dish and garnish with additional grated cheese. (If you choose, bake in a 400° oven until slightly browned and bubbling, about 15 minutes.) Sprinkle with more celery and parsley leaves, chives and potato chips.