In a large saucepan, boil the potatoes until soft. Drain and mash in the pan while still hot.Return the pan to the stove, add the cream and butter, and mix well. Add the peas, red sweet pepper, mustard, and 1 teaspoon of chives. Season with salt, pepper, and nutmeg, and then add the avocado. Keep warm.
In a large frying pan, heat the olive oil over medium heat and cook the fish until lightly browned, about 3 minutes per side. Season with salt and pepper. Garnish with the remaining chives and serve with the mashed potatoes.