Peel the Rooster potatoes and cut them into cubes. Put them in a large saucepan. Cover with cold, salted water and bring to a simmer over medium heat. Cook for 20 minutes, or until soft.
Meanwhile, grease an oven dish with olive oil. Add the fish sticks/crumbed fish and place in the oven for 10-15 minutes until golden brown and crispy.
Drain the potatoes and then mash them (using a potato masher) together with the tartare sauce and fresh spinach. Season to taste with salt and pepper. Set aside.
Finely chop the onion and cut the leek into rings. Heat a non-stick frying pan with olive oil. Fry the onion and leek for a few minutes. Add the frozen peas, cream, and fish stock. Simmer for 6 minutes or until the vegetables are cooked through. Season with salt and pepper
Remove the fish sticks/crumbed fish from the oven and mash them with a fork. Add the vegetable mixture in a layer on top of the fish. Finish with a layer of mashed spinach potato. Drizzle some olive oil on top and place back in the oven. Bake for 10 more minutes until golden brown.
