Meanwhile, mash cooked potatoes with 1/4 cup butter, 1 egg, asparagus puree and salt and white pepper.
Preheat broiler. Cook spinach in large saucepan with remaining butter. Season with salt and pepper. Cover pan and cook on low heat for 5 minutes, stirring once. Drain spinach and place 1/2 in an ovenproof dish.
Heat frying pan and add pike with marinade. Season with salt and white pepper, bring to a boil, cover and poach for 8 minutes. Drain fish and place on bed of spinach. Drizzle with lemon juice and cover with remaining spinach, then 1/2 of potato. Pipe remaining potato decoratively around edge of dish and broil for 6 minutes. Dust with parsley and paprika and serve.