Heat the olive oil in a heavy saucepan. Add the onions and fry gently, stirring frequently until they are soft and translucent. Add the chicken breasts and fry for a minute or two. Add the garlic, ras-el-hanout, saffron, ginger, and pepper and fry for 2-3 minutes until golden. Add the preserved lemon, coriander, parsley and the olives.
Add enough water to just cover the chicken, then bring to the boil. Reduce the heat, cover and simmer on a low heat for about 35 minutes.
For gluten-free diets try serving the tagine with brown rice instead of couscous.