In a large bowl gradually whisk flour into water. Whisk in salt and oil. Let batter stand, covered, at room temperature 1 hour or chilled 1 day.
Preheat broiler and set rack about 5 inches from heat. Oil a 13- by 9-inch flame proof baking pan (preferably with olive oil).
Stir batter and pour into pan. Tear sage leaves into bits and sprinkle sage and pepper over batter. On broiler rack broil farinata 5 minutes, or until top is speckled with brown spots. Reduce temperature to 450°F and bake farinata 5 minutes more, or until set and pulls away slightly from sides of pan.
