In a medium bowl, combine chickpea flour, water, salt, and 3 tablespoons oil until no lumps remain. Let sit 30 minutes to allow chickpea flour to soften.
Meanwhile, arrange a rack in upper third of oven. Place a 10" cast-iron skillet on rack; preheat oven to 450°.
When oven is preheated, remove skillet from oven. Carefully pour remaining 1 tablespoon oil into skillet, add batter, and bake 10 minutes.
Turn on broiler. Broil, watching carefully, until socca is golden brown, 4 to 5 minutes.
Top with thyme (if using). Slice and serve warm.
