Peel and slice the onions as finely as possible. Peel and thinly slice the garlic. Heat a heavy based pan. Tip in the coriander seeds and toast for 2-3 mins till popping and fragrant. Lightly crush in a pestle and mortar.
Keep the pan on the heat. Drizzle in 2 tbsp olive oil. Add the onions and garlic and lower the heat. Add a good pinch of salt. Cook for 10 mins till the onions are soft and melting. Stir every so often.
Pour 1 ltr boiling water into a heatproof jug. Crumble in the stock cube. Peel the turmeric and grate it into the stock. Sprinkle the sugar into the pan with the onions after they have sweated down for 10 mins. Cook for 5 mins till slightly caramelised.
Pour the stock into the caramelised onions. Add 1 cinnamon stick and the toasted coriander seeds. Stir and cover. Cook on a medium-low heat for 20 mins.
Juice the lemon. Taste the soup and add lemon juice to taste. Season. Stir in most of the mint and dill leaves. Ladle into bowls and scatter over a pinch of remaining herbs.