For the fish casserole, boil the potatoes until soft, peel and mash with a little olive oil, salt, pepper and nutmeg. Defrost the spinach and season with finely chopped garlic and salt.
Preheat the oven to 220 °C top and bottom heat. Chop the onions and sauté in olive oil until translucent. Add the soy sauce and simmer for a few minutes.
Add the grated Parmesan cheese, mustard, juice and grated zest of the lemon and season with salt. Cut the fish fillets into cubes.
Brush a baking dish with a little olive oil and spread the spinach in it. Place the fish cubes on top of the spinach and pour the onion sauce over them.
Finally, cover everything with the mashed potatoes and bake for about 35 minutes, until the potato cap is lightly browned. Serve the finished fish casserole sprinkled with parsley.