In a big pan, heat the oil and cook the onion and red pepper until cooled for about 5 minutes. Add the garlic, harissa and sliced vegetables and boil until thickened for about 10 minutes. Add and boil the frozen seafood, about 10 minutes longer.
Place the couscous in a serving bowl, pour over to cover the boiling water, then cover it with a cling film. Stir in most coriander, almonds, lemon zest and juice, and seasoning while the tagine is cooking. Serve the couscous tagine and cut half of the remaining lemon into wedges to squeeze over. Sprinkle over and serve the remaining coriander.