Sift the gram flour and the self-raising flour into a bowl. Add the chilli powder, garam masala and tandoori masala powder, if using, and stir well to combine.
Add the lemon juice, then gradually add enough water to form a smooth batter that is thick enough to coat the back of a spoon. Set aside for 10-15 minutes.
Add all of the remaining ingredients except the sunflower oil to the batter mixture and mix well to coat the potatoes and onions.
Heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
When the oil is hot, carefully lower tablespoons of the batter mixture in batches into the oil and fry for 5-6 minutes, or until the pakoras are crisp and golden-brown and the vegetables have cooked through. Serve immediately.