In a medium bowl, whisk together the chickpea flour, water, 1 tablespoon plus 1 teaspoon olive oil, salt, ground coriander and turmeric.
Cover with plastic wrap and let the mixture rest at room temperature for at least 2 hours.
Preheat the broiler, with the rack set 7 to 8 inches from the element.
Heat 1 teaspoon of olive oil in a large nonstick skillet set over medium-high heat.
Add the onion and cook until starting to brown, about 2 minutes. Stir in the tomato and cook for 3 minutes. Add the garlic and cook for 30 seconds. Stir the vegetables into the chickpea flour batter.
Place a large (10-inch) cast iron skillet in the oven to preheat for 5 minutes.
Using an oven mitt or potholder, carefully remove the cast iron skillet from the oven. Pour in 1 tablespoon olive oil and swirl to coat the pan.
Pour in half of the batter and immediately swirl to coat the bottom of the pan.
Place under the broiler and cook until the top and edges are starting to blacken and blister, 4 to 6 minutes.
Carefully transfer the flatbread to a cutting board, cut into 8 wedges, garnish with cilantro and serve.
Repeat with the remaining 1 tablespoon olive oil, batter and cilantro.