Preheat oven to 425°.
To prepare polenta, combine first 3 ingredients in a medium saucepan; bring to a boil. Gradually stir in cornmeal, stirring constantly. Reduce heat, and simmer 5 minutes or until thick, stirring frequently. Spread mixture into an 8-inch square baking dish coated with cooking spray.
To prepare tomatoes, heat oil in a large nonstick skillet over medium-high heat. Add tomato and the next 5 ingredients (through garlic); sauté 6 minutes or until tomato is tender. Remove from heat; stir in chopped cilantro. Spoon tomato mixture over polenta; sprinkle with cheeses. Bake at 425° for 20 minutes or until lightly browned. Garnish with cilantro sprigs, if desired.