Place the cornmeal and 1/2 teaspoon salt in a large saucepan. Gradually add water, stirring constantly with a wire whisk. Bring to a boil, and reduce heat to medium. Cook 15 minutes, stirring frequently. Spoon the polenta into an 8 1/2 x 4 1/2-inch loaf pan coated with cooking spray, spreading evenly. Press heavy-duty plastic wrap onto surface of polenta, and chill 2 hours or until firm.
Heat oil in a large nonstick skillet over medium heat. Add garlic, thyme sprigs, and rosemary sprig; cook 3 minutes or just until garlic begins to brown. Stir in mushrooms and next 5 ingredients (mushrooms through pepper); bring to a boil. Cover, reduce heat, and simmer 15 minutes, stirring occasionally. Discard the thyme and rosemary sprigs. Add parsley; cook, uncovered, 5 minutes. Remove from heat; set aside, and keep warm.
Invert polenta onto a cutting board; cut crosswise into 12 slices. Place slices on a baking sheet coated with cooking spray. Broil 5 minutes on each side or until golden brown.
Arrange 2 polenta slices on each of 6 individual serving plates. Top each with about 1/3 cup of mushroom sauce and 1 1/2 teaspoons of Parmesan cheese. Garnish with thyme sprigs, if desired.