Heat a large pot on high heat. Add the cumin seeds, coriander seeds, mustard seeds (if using), and desiccated coconut. Toast for 3-4 minutes, until the cumin seeds start to pop and the mixture is fragrant. Remove from the pot, transfer to a pestle and mortar, and grind until smooth. If you don’t have a pestle and mortar, let the mixture cool, then place it in a zip-lock bag and gently bash with a mug or rolling pin until ground to a powder.
Peel and roughly chop the spring onions, garlic, and ginger. Dice the chilli, including the seeds if you prefer more heat, or remove the seeds for less spice. Chop the sweet potato, red pepper, broccoli, and aubergine into bite-sized pieces for quicker cooking.
Put the large pot back on high heat. Once hot, add the chopped spring onions and fry for 4 minutes, stirring continuously. If the mixture starts to stick, add ½ tsp of water and stir to deglaze the pot. Add the garlic, ginger, and chilli, and fry for another 2 minutes, stirring regularly. Add 1 tsp of water if necessary to prevent sticking. The spring onions should be browning, and the garlic should turn golden.
Add the chopped broccoli, sweet potato, aubergine, and red pepper, along with the salt, curry powder, toasted spices, ground black pepper, tamari, and 50ml of the vegetable stock. Stir well, cover the pot, reduce the heat to medium, and cook for 10 minutes, stirring occasionally.
Add the coconut milk, red lentils, remaining vegetable stock or water, and tamarind pulp (or lime juice and sugar). Stir well, increase the heat to high, and bring the mixture to a boil with the lid on.
Once it reaches a boil, reduce the heat to a simmer and cook for 25 minutes, stirring regularly to prevent the lentils from sticking. Add the baby spinach during the last 5 minutes of cooking, if using.
Check the seasoning and adjust with more salt, pepper, or lime juice if needed.
Roughly chop the fresh coriander or parsley. Remove the pot from heat and stir in the fresh herbs just before serving.
Garnish each bowl with a generous dusting of pickled ginger, if desired.