Assemble, prepare, and measure ingredients. Peel and cube potatoes. Separate cauliflower into florets. Chop spinach. Dice onion. Peel and dice carrots. Grate Cheddar. Juice lemon. Chop parsley. Preheat oven to 450°F/230°C.
Bring a large saucepan of salted water to a boil and cook potatoes for 10 minutes, or until tender. Add cauliflower and cook for another 5 minutes. Remove vegetables with a slotted spoon, reserving cooking water. Spread veggies out to dry. When they stop steaming, mash with 1 tablespoon of olive oil. Season with nutmeg, and salt and pepper. Set aside.
Place spinach in a sieve and pour reserved warm vegetable cooking water over it. When spinach is cool enough to handle, squeeze out any excess water. Set aside.
In a skillet over medium-low heat, warm 1 tablespoon of oil. Add onions and carrots and sauté for 5 minutes, or until onions are translucent and carrots are tender. Add cream and bring to a boil for 30 seconds. Remove from heat and stir in Cheddar, 1 tablespoon of lemon juice, parsley, and dry mustard.
In a 13- × 9-inch/2 L baking dish, combine spinach and fish. Pour cheese-vegetable sauce over fish and top with mashed potatoes and cauliflower. Brush with olive oil and bake for 20–25 minutes, or until topping is well browned. Remove from oven and allow to rest for 10 minutes. To serve, spoon fish pie onto four plates.