Place the split peas in a pot with the water, onions, ginger, garlic, chilli, turmeric and ground coriander. Bring to a boil and simmer without a lid for 30-45 minutes or until the split peas are soft. When cooked, add salt and spinach and simmer 5 minutes.
In a separate frying pan, heat the oil to medium high, then add the cumin and mustard seeds and cook until they start to pop. Pour them into the soup. Cover and let sit for 2 minutes. Add lemon juice and fresh coriander, stir and serve.