Peel and slice the onions as finely as possible. Peel and thinly slice the garlic. Heat a heavy-based pan for 2 mins. Tip in 1 tsp coriander seeds and toast for 2-3 mins till popping and fragrant. Tip out of the pan and lightly crush in a pestle and mortar, or in a bowl with the bottom of a jar.
Put the pan back on the heat. Add 2 tbsp olive oil, 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. Set aside.
After the onions have sweated for 10 mins, sprinkle 1 tbsp sugar into the pan. Cook for 5 mins till the onions are slightly caramelised.
Pour the stock into the caramelised onions. Add 1 cinnamon stick and the toasted coriander seeds. Stir and cover. Let 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.