Preheat oven to 375F.
Bring a large pot of water to a boil, season with 1 tablespoon salt and add the potatoes. Boil until fork-tender, around 15 minutes. Drain and let sit 5 minutes to allow the steam to escape. Add 1 tablespoon olive oil, season with salt and pepper to taste, and mash until chunky. Set aside.
Heat a saute pan over medium-high heat, add 2 tablespoons olive oil and heat until shimmering. Add the celery, carrot, garlic and parsley stems. Season with a pinch of salt and cook, stirring, until slightly softened, around 8 minutes.
Add the ghee and stir until melted. Add the arrowroot starch and cook, stirring, until dissolved, 2-3 minutes. Add the coconut milk, mustard, hot sauce, bay leaves, and black pepper to taste. Cook, stirring regularly, until thickened and reduced by half, around 8 minutes. Remove from heat and discard the bay leaves. Stir in the peas and reserved parsley leaves. Taste for seasoning and adjust as desired.
Scatter the fish and shellfish in the bottom of a medium-sized baking dish. Pour over the cream sauce and top with the reserved mashed potatoes. Drizzle the top with 1 tablespoon olive oil and bake until bubbling and golden brown, 20-25 minutes. Remove from oven and let rest 5 minutes before serving.