How to Make It
Whisk together flour and 2 cups water in a large bowl until no lumps remain. Cover and let stand in a warm place until small bubbles form over entire surface of flour mixture and mixture has lightly fermented, at least 4 hours. (Inside a closed oven with the oven light turned on is perfect place to do this.)
Preheat oven to 500°F. Using a mandoline or very sharp chef’s knife, thinly slice fennel bulb to equal 3 cups. Chop fennel fronds to equal 2 tablespoons. Heat 1 tablespoon of the oil in a 8-inch cast-iron skillet over medium. Add fennel bulb, pepper, and 1/4 teaspoon salt, and cook, stirring often, until softened and lightly golden, 10 to 12 minutes. Transfer to a plate.
Heat 3 tablespoons oil in skillet over high. Stir remaining 1/2 teaspoon salt into batter. Add a ladleful (about 3/4 cup) of chickpea batter to center of skillet in a steady stream. (The batter should be no thicker than 1/4 inch.) Gently swirl skillet to allow oil to come up over the sides of batter. Top batter with 1/2 cup cooked fennel, 1/2 tablespoon chopped frond fronds, and 1/4 teaspoon fennel seeds. Cook on high until small holes start to form around edges of batter and oil is bubbling up through holes, about 1 minute.
Transfer skillet to preheated oven, and bake until bottom is browned and crispy and top is set, 12 to 15 minutes. Remove socca from skillet, and cut into quarters. Repeat procedure 3 times with remaining batter, cooked fennel, fennel fronds, and fennel seeds.