Spicy potatoes and cauliflower are made into a tasty main course by the addition of lentils.
In salted water, separately parboil the sweet potato and cauliflower pieces until almost cooked (5-10 min). Set aside.
Heat the oil in a large frying pan, saute pan or wok then add the onion and cook over a medium heat for 10 min, stirring regularly and making sure it doesn't burn.
Add the garlic and continue to cook, stirring regularly, for a further 5 min until the onion and garlic are softening and starting to brown.
Add the chilli, ginger, cumin, mustard and fennel seeds and the curry leaves. Stir and cook for 5 min.
Add the potatoes, cauliflower, cooked lentils and fry until spots on them are starting to brown a little (5 min).
Add the turmeric, ground coriander, fenugreek and garam masala. Stir.
Add the tomatoes, tomato puree, sugar, salt, 2 tbsp of the chopped coriander and 200ml of water. Stir, scraping up any spices stuck to the bottom of the pan, bring to the boil and immediate turn down the heat to low.
Put a lid on and simmer until the vegetables are cooked through, but still hold their shape (15-20 min).
If using, add the coconut milk and simmer with the lid off until it's reduced (5 min).
Check the seasoning: stir in a little more salt or garam masala if you think it needs it.
Just before serving, stir in the remaining 2 tbsp of chopped coriander and the lemon juice.