Firstly, put the cornflour in a bowl, then stir in the remaining ingredients to make a thick batter. Set aside.
Heat the oil for deep-frying in a deep-fat fryer or a deep, heavy-based saucepan.
Dip the broccoli and cauliflower florets in the batter, letting the excess drip off, and deep-fry for about 3 minutes, until crisp and golden brown. Transfer to kitchen paper to drain and keep hot.
Heat the oil in a large kadhai, wok or heavy-based frying pan until smoking point. Add the curry leaves and garlic, onion, chillies and peppers and stir-fry on a high heat for 2 minutes.
Add the fried broccoli and cauliflower, ginger, salt and sugar and continue stir-frying for 2 minutes.
Squeeze in the lemon juice and sprinkle with the fresh coriander.
Serve with steamed rice.