In a medium bowl, combine flour, water, olive oil, and salt. Whisk until smooth. Let sit for at least 30 minutes.
Meanwhile, set oven to broil and place cast iron skillet in oven for 5 minutes
Remove skillet from oven, add 2 tsp olive oil and swirl to coat. Pour batter into the pan.
Return skillet to oven, placing under broiler element. Broil socca for about 5 minutes, until browned and edges are crisp. Reduce oven temperature to 400°F and continue baking until fully set, about 8-10 minutes.
Use a spatula to remove socca from pan and place on cutting board. Cut into wedges, sprinkle with salt, pepper, sumac, and drizzle with additional olive oil.
