1. To make Mostarda: Bring sugar and 1/2 cup water to a boil in medium saucepan. Add cranberries, reduce heat to medium, and cook 5 minutes, or until cranberry skins split. Transfer cranberries to plate with slotted spoon. Add onion, apricot preserves, and red pepper flakes to pan; reduce heat to medium-low, and simmer 10 minutes. Return cranberries to pan, cook 8 minutes more, or until mixture is thick, stirring often. Cool, then stir in mustard and vinegar. Cover, and chill.
2. To make Polenta Squares: Line baking sheet with foil. Purée corn and 1/2 cup water in blender until smooth. Pour into large saucepan with 3 cups water and 1 tsp. salt, if desired. Bring to boil. Gradually add polenta; cook 2 minutes, stirring constantly. Reduce heat to low. Cook and stir 18 minutes, or until very thick.
3. Scrape polenta onto foil in mound. Let rest 1 minute. Spread polenta to 7 1/2-inch square with metal spatula. (Use a ruler to make straight edges.) Cool, then cover with plastic wrap. Chill 2 hours (or up to 2 days), until firm. Cut polenta into 25 1 1/2-inch squares.
4. Coat large nonstick skillet with cooking spray. Cook Polenta Squares in batches over medium-high heat 4 minutes per side, or until lightly browned.
5. Spread prepared mustard on each square. Top each with 1 tsp. Mostarda, and sprinkle with green onions (if using).