How to Make It
Divide ½ cup of the oil between 2 large pots and heat over medium heat. Divide the onions, carrots, parsnips, and celery between the pots and cook, stirring occasionally, until tender, 18 to 20 minutes. Dividing evenly, sprinkle with the flour and cook, stirring, for 1 minute (do not let darken). Divide the wine between the pots and simmer for 1 minute.
Divide the cream between the pots; season each with 1 teaspoon salt and ½ teaspoon pepper and bring to a boil. Remove from heat and, dividing evenly, stir in the parsley. Divide the vegetable mixture among four 8-inch square baking dishes and let cool.
Meanwhile, place the potatoes in a large pot and add enough cold water to cover. Bring to a boil and add 1 tablespoon salt. Reduce heat and simmer until tender, 15 to 18 minutes. Reserve 3 cups of the cooking water, drain the potatoes, and return them to the pot. Add 1 cup of the remaining oil, 1 ½ teaspoons salt, 1 teaspoon pepper, and 1 cup of the reserved cooking water and mash (adding more cooking water, if necessary) until smooth.
Dividing evenly among the baking dishes, nestle the fish and shrimp in the vegetable mixture. Top with the potato mixture.
In a small bowl, combine the bread crumbs and the remaining ¼ cup of oil. Dividing evenly, sprinkle over the potatoes.
To eat tonight: Bake the casserole on a rimmed baking sheet at 375° F until golden brown, 20 to 25 minutes.
To freeze and cook later: Freeze the unbaked casserole, tightly sealed, for up to 3 months. To cook, thaw the casserole and bake, uncovered, on a rimmed baking sheet at 375° F until heated through and golden brown, 40 to 45 minutes.