Preheat the oven to 200°C (392°F).
Peel and finely chop the spring onions, garlic, ginger, and deseed the chilli if you prefer less spice.
Chop the cauliflower into small bite-sized pieces and do the same with the potatoes.
Fill a pot with boiling water, add a pinch of salt, and boil the cauliflower and potatoes for about 10 minutes. Drain and rinse them under cold water.
Place the boiled cauliflower and potatoes on a baking tray, drizzle with 2 tablespoons of oil, and sprinkle a generous pinch of salt. Bake for 10–15 minutes until the cauliflower starts to char and the potatoes become soft.
Heat a large non-stick frying pan over high heat and add 1 tablespoon of oil. Once the oil is hot, add the chopped spring onions, ginger, garlic, and chilli. Fry for about 3 minutes.
Add the black mustard seeds, turmeric, cumin seeds, and garam masala to the pan, and fry the spices for 1 minute.
Pour in the coconut milk and vegetable stock, bringing the mixture to a boil. Then reduce the heat and let it simmer.
Add the baked cauliflower and potatoes to the pan, stirring everything together. Let it cook for about 5 minutes until the sauce starts to thicken.
Add the lemon juice and tamari to the pan, seasoning with salt and black pepper to taste. Adjust the balance of flavours with more seasoning if needed.
Before serving, garnish with a dollop of coconut yogurt and fresh coriander leaves.