How to Make It
Heat oven to 375° F. Heat 2 tablespoons of the oil in a large pot over medium heat. Add the onion, carrots, parsnips, celery, and ½ teaspoon salt and cook, stirring occasionally, until tender, 15 to 18 minutes. Sprinkle with the flour and cook, stirring, for 1 minute (do not let darken). Add the wine and simmer until nearly evaporated.
Add the cream, ½ teaspoon salt, and ¼ teaspoon pepper and bring to a boil. Remove from heat, stir in the parsley, and transfer to an 8-inch square baking dish.
Meanwhile, place the potatoes in a medium saucepan and add enough cold water to cover. Bring to a boil and add 1 teaspoon salt. Reduce heat and simmer until tender, 15 to 18 minutes. Reserve ¾ cup of the cooking water, drain the potatoes, and return them to the pot. Add ¼ cup of the remaining oil, ½ teaspoon salt, ¼ teaspoon pepper, and ¼ cup of the reserved cooking water and mash (adding more cooking water, if necessary) until smooth.
Nestle the fish and shrimp in the vegetable mixture and top with the potato mixture.
In a small bowl, combine the bread crumbs and the remaining 1 tablespoon of oil and sprinkle over the potatoes. Place the dish on a rimmed baking sheet and bake until golden brown, 20 to 25 minutes.