Fill a cast iron skillet with 1-inch-deep oil. Place over high heat and heat to 350℉.
In a large bowl, mix together flour, cilantro, curry powder, salt, baking powder, and chili flakes. Stir in lukewarm water until batter is well combined.
Add cauliflower and onion to batter. Use a spatula or your hands to mix until vegetables are evenly coated.
Working in batches, use a large serving spoon to drop portions of the batter-coated vegetables into the hot oil and cook until golden brown, about 1 to 2 minutes per side. Do not overcrowd the skillet. Let drain on paper towels before serving.