Step 1Heat 2 tablespoons oil in large skillet over medium-high heat. Add onions and saute until tender, 3 to 4 minutes. Add stock and bring to a boil. Add polenta slowly, whisking constantly. Reduce heat to medium and continue to cook, stirring constantly, until polenta comes away from sides of pan, about 20 minutes. Season with salt and pepper.
Step 2While still hot, spread polenta about 1-inch thick onto an oiled baking pan. Cool, cover and refrigerate until cold and firm, several hours or overnight. Using a (2-inch) round scalloped cookie cutter, cut polenta into rounds and transfer to a large platter.
Step 3Heat 2 tablespoons oil in nonstick skillet and brown polenta rounds, turning occasionally, until brown and crispy on both sides, 8 minutes. Drain on paper towels and repeat with remaining polenta rounds, adding additional oil as needed. Serve immediately or reheat just before serving.