Step 1Combine the flour and salt in a medium bowl. Whisk in the water and 1 tablespoon olive oil until smooth. Cover the batter loosely and let stand at room temperature preferably for 4 to 8 hours.
Step 2Heat the oven to 450 degrees. Toss the red bell pepper and fennel with the garlic and 1 tablespoon olive oil in a well-seasoned 10-inch cast iron skillet. Roast the peppers and fennel in the oven until tender and browned, about 10 minutes. (Maintain oven temperature.) Using tongs, remove the vegetables from the skillet. Inspect the skillet to be sure that there are no spots where the vegetables stuck. Wipe clean with a paper towel if necessary. Return the skillet to the oven and heat until very hot, about 15 minutes. Working quickly, pour the remaining 1 tablespoon olive oil into the skillet and carefully swirl to evenly coat the bottom of the skillet. Distribute the vegetables evenly over the bottom of the skillet. Whisk the batter and pour it over the vegetables. Sprinkle the surface of the socca evenly with the olives and rosemary and bake until golden brown and set, about 15 minutes. Cut into wedges and serve immediately.