How to Make It
Prepare the crust: Toss together 1 cup cubed butter and 2 cups flour in a large bowl until butter is coated in flour; transfer coated butter and flour to a food processor. Add sugar, cayenne pepper, and 1/2 teaspoon salt; process until large clumps form, about 20 seconds. Slowly add up to 1/2 cup water, 1 tablespoon at time, while pulsing, adding just enough water to form a dough. Using hands, shape into a loose ball. Dust dough with flour, wrap in plastic wrap, and chill 1 hour or up to 2 days.
Preheat oven to 400°F. Unwrap dough, and turn out onto a lightly floured surface. Roll to 1/4-inch thickness, and cut out a 12-inch circle. Transfer dough circle to a baking sheet, and bake in preheated oven until golden brown, about 20 minutes.
Prepare the filling: Toss together fennel, carrots, celery, celery root, onion, potatoes, thyme, garlic, and olive oil in a large bowl. Add 1/2 teaspoon of the salt; toss lightly. Spread vegetables in a single layer on a baking sheet, and bake at 400°F until cooked through, about 20 minutes. Discard thyme sprigs and garlic cloves.
Melt 3 tablespoons butter in a 12-inch straight-sided skillet over medium; whisk in 3 tablespoons flour. Cook, whisking constantly, until fragrant and the roux takes on a blonde color, about 3 minutes. Add mace, onion powder, garlic powder, bay leaf, and 1 teaspoon of the salt. Gradually whisk in milk. Reduce heat to low, and cook until thickened, about 15 minutes.
Sprinkle fish, scallops, and shrimp with remaining 1 teaspoon salt, and let stand 5 minutes. Gently stir seafood, vegetables, and smoked paprika into sauce mixture in skillet, and cook until fish is cooked through, about 10 minutes. Discard bay leaf. Stir in sherry; sprinkle parsley over mixture. Place the crust over the pot, trim with kitchen shears, and serve.