Heat 2 ½ tablespoons of oil in a large skillet on medium heat.
Add the onions and saute for 5-7 minutes until translucent.
Add the scallions and saute an additional 2-3 minutes until soft. Remove the pan from the heat.
Stir the herbs into the pan and transfer the onion mixture to a wide dish and set aside to cool, about 10 minutes.
Place the flour, baking powder, salt, and pepper in a mixing bowl and whisk to combine.
Pour 1 ¾ cup of the sparkling water into the bowl while whisking continuously until a thick, smooth batter forms.
Gently fold in the onion mixture. Mix in additional sparkling water one tablespoon at a time if the batter seems too dense. It should be thick but flatten a little when spooned into the hot pan without spreading out very much.
Heat a small amount of oil on medium in a large cast iron or nonstick pan.
When the oil is very hot, spoon the batter into the pan in scant ¼ cup portions.
Cook for 2 minutes per side until the pancakes are golden brown.
Add a little more oil to the pan and repeat the previous two steps until all of the batter is used.