Heat the oven to 180C. Put the cashews in a heatproof bowl, add freshly boiled water to cover and leave to soak for 10 minutes. Drain, add 250ml water and blend smooth.
Break the cauliflower into bite-sized florets and put in a large bowl. Pour over just enough oil to coat, add half a teaspoon of salt, toss, then transfer to a baking tray. Roast for 30-40 minutes, until tender and nicely browned in places.
In the meantime, heat three tablespoons of oil in a pan over a medium flame, then fry the onions for 12-15 minutes, until soft and browning. Add the garlic and ginger, fry for 3-4 minutes, then stir in the cardamom, turmeric, pepper and garam masala. Add the sugar, 1 1/2 tsp of salt and the cashew paste, then cook for about 10 minutes, until the sauce turns a rich golden colour. Add enough milk to thin the sauce to the consistency of pouring yoghurt, add frozen veggies, and leave to simmer gently for 10 minutes. Season to taste and turn off the heat. Mix in the cauliflower and serve