Preheat the oven to 190°C/fan 170°C/gas mark 5.
In a large casserole dish with a tight-fitting lid, heat the olive oil over a medium heat. Add the garlic, onions and salt and cook for 10-15 minutes, or until the onions are softened and golden brown.
Stir in the tomato puree and cook for a further minute.
Add the chicken, chilli powder, cumin, coriander, cinnamon, turmeric and nutmeg to the dish and mix to coat.
Pour in the yogurt and sugar and stir to combine.
Bring to a boil, then reduce the heat to low and simmer for 15 minutes, or until the chicken is cooked through.
While the chicken is cooking, rinse the rice in a colander until the water runs clear.
Add the rice, water, salt, cinnamon stick and saffron to a large saucepan. Bring to a boil, then reduce the heat to low and simmer for 10 minutes, or until the rice is just cooked through.
Drain the rice and discard the cinnamon stick.
Spread the rice over the chicken in the casserole dish and cover with the lid.
Bake in the preheated oven for 20 minutes, or until the rice is heated through.
Remove from the oven and fluff the rice with a fork.
Scatter over the spring onions and serve with lemon and lime wedges.