Combine gram flour, cumin, cayenne pepper, turmeric, and salt in a bowl. Add 2 tablespoons of the hot oil and mix well. Add water, a little at a time, until a very thick batter forms. Stir in cilantro and spinach.
Dip potatoes and cauliflower in batter and add to the hot oil in batches without crowding. Turn and fry until golden brown, 3 to 5 minutes. Add onions to remaining batter all at once; scoop onion-batter into the oil and fry until golden, 3 to 5 minutes.
