Peel and thinly slice the onion. Peel and grate or crush the garlic. Rinse the pepper and halve it. Scoop out the seeds and white bits. Slice it. Grate or pare the zest from the lemon. Juice it. Rinse and halve the cherry tomatoes.
Chop the salmon fillets into chunks. Pop to one side. Fill the kettle and boil it.
Warm a glug of oil in a pan. Add the onion. Fry over a low heat for 6-8 mins till glossy and soft but still pale. If it starts to brown, turn the heat down.
Tip the couscous into a bowl and season with a pinch of salt and pepper. Pour in 250ml hot water from the kettle. Stir, cover with a plate and leave to stand for 5 mins till the water has been absorbed.
Add the garlic. Measure out 1 tsp of the ras el hanut. Add to the onion. Cook and stir for 1 min till fragrant. Add the pepper, lemon zest, olives and cherry tomatoes. Pour in 150ml hot water from the kettle. Cover. Simmer for 5 mins till the tomatoes start to soften.
Rinse the parsley. Pat dry with kitchen paper. Pick the leaves of the stalks. Roughly chop the leaves. Stir most of the parsley into the couscous.
Gently stir the chunks of salmon into the tagine sauce. Cover and simmer for 2-3 mins till the fish is cooked through and just starting to flake.
Add a splash of lemon juice to the tagine. Taste and add more lemon juice, salt or pepper if you think it needs it. Serve the tagine with the couscous, sprinkled with the last of the parsley.