Instructions

 Line 8½ by 4½-inch loaf pan with parchment paper and lightly coat with vegetable oil spray. Cook 4 teaspoons oil and three-quarters of garlic in 8-inch nonstick skillet over low heat, stirring often, until garlic is golden and fragrant, about 10 minutes. Off heat, stir in rosemary; set aside.
 Bring water to boil in large saucepan. Reduce heat to low and stir in salt. Slowly add polenta while whisking constantly in circular motion to prevent clumping. Continue to cook, stirring often, until polenta is soft and smooth, 3 to 5 minutes. Off heat, stir in oil-garlic mixture and ⅛ teaspoon pepper.
 Pour polenta into prepared pan, smooth top, and let cool to room temperature, about 2 hours. Wrap pan tightly in plastic wrap and refrigerate until polenta is very firm, at least 2 hours or up to 24 hours.
 Combine olives, tomatoes, vinegar, remaining 1 tablespoon oil, remaining garlic, and remaining ⅛ teaspoon pepper in bowl; set aside.
 Run small knife around edge of polenta, then flip onto cutting board; discard parchment. Trim polenta loaf as needed to create uniform edges. Cut loaf in half lengthwise, then cut each strip crosswise into 6 pieces. Slice polenta pieces in half to form ¼-inch-thick squares. (You should have 24 squares.)
 Adjust oven rack 3 inches from broiler element. (If necessary, set overturned rimmed baking sheet on oven rack to get closer to broiler element.) Place rimmed baking sheet on rack and heat broiler for 10 minutes. Carefully remove sheet from oven. Spray vegetable oil spray evenly on hot sheet and arrange squares in single layer. Broil polenta until spotty brown and crisp, 8 to 10 minutes. Transfer polenta to serving platter, top each square with olive mixture, sprinkle with basil, and serve.