Peel the onions and slice thinly. Put the onions into a large pan with the olive oil and add some salt and pepper. Cover the pan and cook gently for at least 15 minutes, stirring occasionally.
Add the turmeric, fenugreek and mint and cook for a few minutes more, uncovered.
Add the stock and cinnamon stick then bring to the boil, Reduce the heat and simmer for at least 20 minutes.
Add the juice of half a lemon and season with salt and pepper. Add the fresh herbs if using and serve.