Working in 2 batches, pulse the herbs until finely chopped but not wet and mushy, 15-20 pulses (or chop by hand); transfer to a bowl, add the chives, and set aside. 

Heat 2 tablespoons of the oil in a 10-inch nonstick ovenproof skillet over medium heat. Add the onions and a generous pinch of salt and pepper and cook, stirring, until tender and very lightly golden, 9 to 10 minutes. Add the garlic and cook, stirring, 1 minute; transfer the onion mixture to a plate to cool. 

Preheat the oven to 500°F. Whisk the eggs, salt, cayenne, baking powder, and pepper in a large bowl, then stir in the herbs until incorporated into a thick mixture. 

Heat the remaining 4 tablespoons oil over medium heat, then pour in the egg mixture and cook, covered, until kuku is set but the center is still slightly soft, 8 to 9 minutes.

Transfer the skillet to the oven and cook until the center is set, 2 minutes. Use oven mitts to remove the skillet from the oven, cool slightly, then tilt the skillet to move the kuku to a serving platter. Serve warm or at room temperature.