Put oil in a 9- to 10-inch ovenproof frying pan. Add sage leaves and mix to coat with oil, then lift out leaves and put in a small bowl.
Peel and thinly slice onion. Put onion in frying pan over medium-high heat and stir often until golden and sweet-tasting, about 10 minutes.
Meanwhile, in a bowl, whisk garbanzo flour and 1/4 teaspoon salt with 1 1/2 cups water until smooth.
Reduce heat under onions to medium-low. Push onion slices to center of pan and pour garbanzo mixture around them, then lift onions so batter can flow under them. Sprinkle socca with sage leaves. Cook until socca feels dry when lightly touched and is browned on the bottom (lift carefully with a spatula to check), 12 to 14 minutes.
Broil 6 to 8 inches from heat until top is lightly browned, 3 to 4 minutes.
Cut into wedges and serve with a wide spatula. Add salt to taste.
Canned option: If garbanzo flour isn't available, omit it from preceding recipe. Instead, drain 1 can (15 1/2 oz.) garbanzos, reserving 6 tablespoons of the liquid. Whirl garbanzos, reserved liquid, and 1 tablespoon all-purpose flour in a blender until very smooth. Use in step Mixture will still feel moist on top when browned on bottom, step 4, and will take 3 to 4 minutes longer to brown when broiled, step
Nutritional analysis per appetizer serving.