Heat oil in a large saucepan over medium-low heat. Add onion and sauté until softened. Add sweet potato and sauté for 5 minutes. Add grated ginger and garlic, stir and reduce heat to low.

Finely dice chilli and add to saucepan with lentils, cumin, ground coriander, turmeric and ground ginger. Add tomatoes and 4 cups of water and stir well to combine. Increase heat to a boil, then reduce until mixture is at a simmer. Cook for 20 minutes uncovered, then add cauliflower, stir and cook for a further 10 minutes.

Season with salt to taste and continue to simmer until the mixture has thickened, about 10 minutes. Use a stick blender to give a quick blitz to lentils and sweet potato.

Serve dhal in bowls, sprinkled with fresh coriander.