Chicken tagine with olives and preserved lemons

Recipe from the Guardian

https://www.theguardian.com/lifeandstyle/wordofmouth/2013/mar/13/how-make-perfect-chicken-tagine

Ingredients

3tbsp olive oil
2 red onions, thinly sliced lengthways
3 garlic cloves
2tsp ground ginger
½tsp saffron, in a little warm water
1tsp cinnamon
Juice of ½ lemon
2 small preserved lemons
2tbsp chopped parsley
Small bunch of fresh coriander
6 chicken thighs
3tbsp violet olives