Combine sour cream and yogurt in a small bowl, stirring with a whisk. Cover and refrigerate.
Heat oil in a medium saucepan over medium heat; add onion. Cook 5 minutes or until tender, stirring frequently; add polenta. Cook 1 minute, stirring constantly. Add water and salt; stir with a whisk until polenta begins to absorb water (about 2 minutes). Reduce heat to low; cover and cook polenta mixture 25 minutes, stirring occasionally. Pour polenta into an 11 x 7-inch baking dish coated with cooking spray. Refrigerate, uncovered, 30 minutes or until cool.
Preheat oven to 400°.
Cut the polenta into 8 pieces with a 2-inch round cutter, and discard excess polenta. Arrange rounds on a baking sheet coated with cooking spray. Bake at 400° for 40 minutes, turning after 20 minutes. Cool completely on a wire rack.
Arrange 2 polenta rounds on each of 4 plates; top each serving with 5 raspberries and 3 fig slices. Drizzle 2 tablespoons maple syrup over each serving; top each with 1 tablespoon sour cream mixture. Garnish with rosemary sprigs, if desired.