Put the dates in a food processor or blender with 1–2 tablespoons water and process to form a smooth purée. (If the dates are not soft they will need to be soaked in water for several hours first.)
Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion and garlic and cook for 2–3 minutes to soften a little. Add the harissa and trout chunks and stir to coat well.
Add the date purée and pour in enough water to cover the fish. Season with salt and pepper. Cover and cook gently for 30 minutes until the fish is cooked through. Stir in most of the coriander and garnish with the remainder. Serve with couscous, if liked.