If you plan to serve these within 20 minutes of cooking, preheat your oven to 200°In a medium bowl, sift together the cornmeal, salt, and sugar. Put the buttermilk and the onion in a blender and puree till it’s a homogenous liquid. Pour that plus 1/2 cup of the water into the cornmeal mixture and whisk to combine.
You’re looking for something akin to slightly loose pancake batter — a batter that, when you drop it into the skillet, spreads on its own, bubbles up around the edges, splatters a little. If you need to add more water to accomplish this, add the remaining water increments.
Heat 1 tablespoon of the oil in a 10-inch cast-iron skillet over medium heat. Spoon 2 tablespoons of batter onto the edge of the pan to form 1 corn cake. If the batter sizzles a little, the pan’s ready. Continue to drop the batter around the perimeter of the pan, finishing off with one in the middle. Make sure you get as many of them in there as you can without letting them touch. Lower your heat slightly and cook on one side for about 3 minutes. When they’re brown on the cast-iron side and little bubbles are shooting up through the center of batter, flip and cook an additional 3 minutes. Transfer the browned hoecakes to a baking sheet and hold them in the oven till you’re ready to eat. Add another tablespoon of oil and continue with the next batch.
If you, like my grandma, want to make these ahead and serve them a few hours later, warm them in a 375°F oven for 12-15 minutes. Do not use a microwave. The results will disappoint.