Heat water, after salting it first, in a medium-sized pot and leave it to boil. Add the couscous and leave to moderate heat for 7 to 8 minutes. Drain the couscous and let it cool down. In the meantime, in a small bowl, mix lemon juice with olive oil. Add salt and pepper.
In a large bowl, mix couscous, parsley, coriander, mint, tomatoes and onions. Add the mixture of lemon juice and olive oil and salt and pepper. Leave it for about 20 minutes before serving to marinate.