In a large casserole dish, heat the olive oil over a medium heat. Add the onions and cook for 8 minutes until softened. Add the garlic, cook for a further minute, then set aside in a bowl.
Turn up the heat, add the chicken thigh fillets to the dish and cook for a few minutes on each side until golden.
Return the onions and garlic to the casserole. Add the ras el hanout, cinnamon, ground ginger, honey, chopped tomatoes and olives, along with 150ml water. Cover and cook for 20 minutes, then remove the lid and cook for a further 10 minutes until the chicken is cooked through with no pink remaining and the sauce has thickened.
Stir a little of the harissa into the tagine to taste.