Citrusy Breakfast Polenta Cake

Name: Citrusy Breakfast Polenta Cake
Ingredients: 2 tablespoons granulated sugar; 2 tablespoons finely grated lemon zest; 1 1/2 cups all-purpose flour, divided; 1 cup coarse-ground cornmeal; 1/4 teaspoon fine salt; 2 large egg yolks, lightly beaten; 2 cups buttermilk; 1/2 cup whole milk; 4 tablespoons (1/2 stick) unsalted butter, melted and cooled; Cooking spray or butter, for coating the baking dish; 1 teaspoon baking powder; 1 teaspoon baking soda; 2 large egg whites, at room temperature; 3/4 cup lemon curd; 2 lemons , peeled, sliced into wheels, and seeded; 1 navel orange, peeled, sliced into wheels, and seeded; 1 Cara Cara orange, peeled, sliced into wheels, and seeded; 1 grapefruit , peeled, sliced into wheels, and seeded; 2 tablespoons coarse sugar