Corn and Onion Focaccia

In a large bowl, sprinkle yeast over water; let stand about 5 minutes. Stir in salt, 1 Tbsp. oil, cornmeal, and 1 cup flour. Beat with a mixer until dough is stretchy, 3-5 min. Stir in 1 more cup flour. Coat a board with remaining flour. Knead dough on board until smooth and elastic. Add more flour if dough sticks. Place dough in a bowl, cover, let rise in warm spot, about 45 minutes. Punch dough down. Pour remaining oil in 10" x 15" baking pan. Turn dough in oil to coat, then press and pat dough evenly to fill pan. Sprinkle with corn and sage; press firmly into dough. Separate onion slices into rings and mix with lemon juice. Scatter onion rings over dough and press down gently. Cover pan and let dough rise, abut 30 minutes. Bake uncovered, on lowest rack in 400 degree oven until dough is well browned on edges and bottom, about 35 minutes. Sprinkle with cheese and bake 2 more minutes. Serve hot, warm, or at room temperature. Cut into squares. Serves 12.