Place 12" skillet or pizza pan in oven; preheat oven to 450°F.
Sift chickpea flour into large bowl. Stir in salt and pepper, then whisk in water until there are no lumps. Add 2 tbsp olive oil. Cover and let stand at least 30 minutes (or up to 12 hours). Batter should be about as thick as heavy cream.
If using onion and/or rosemary, stir them into the batter. Pull preheated pan out of oven and add 2 tbsp olive oil; swirl to coat pan. Pour in batter and bake 12-15 minutes, until firm and edges are set. If top looks dry, brush with 1-2 tbsp more olive oil; place pan under broiler until top is spotty brown. Cut into wedges and serve hot.