Lightly spray a 9-by-13-inch baking dish with cooking spray. In a large saucepan, bring the water and 2 tablespoons of the olive oil to a boil. Gradually whisk in the polenta and cook over low heat, whisking, until thick, about 5 minutes; season with salt and pepper. Scrape the polenta into the baking dish and smooth the top. Cover with plastic wrap and refrigerate until firm, 2 hours.