Place an oven rack on lowest position and preheat to 500°F.
On a lightly floured surface, roll dough into a 14-inch round. Generously sprinkle a baking sheet with cornmeal. Place dough on sheet. Bake until crisp, about 12 minutes.
Heat oil in a nonstick skillet on medium-high. Add onion, salt and pepper. Cook, stirring, until golden. Add 1/4 cup water, cover and cook on low until soft. Stir in spinach; spoon on crust, leaving a 1/2-inch border. Crumble cheese on top.
Bake for 5 minutes. Cut into 8 slices. Serve.