Bring first 4 ingredients to a boil in a medium saucepan over medium-high heat. Add quince; reduce heat to medium-low (mixture will just barely simmer), and cook 45 minutes or until quince is tender. Remove from heat, and cool to room temperature. Strain quince mixture through a sieve into a bowl, reserving quince and liquid; discard rosemary. Return liquid to pan; bring to a boil over medium-high heat. Cook 10 minutes or until reduced to about 1/4 cup; keep warm.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Arrange polenta slices in pan in a single layer; sprinkle with pepper. Cook for 8 minutes or until lightly browned; turn and cook an additional 8 minutes or until lightly browned. Place polenta rounds on a baking sheet. Top each round with 1 tablespoon quince and about 1 teaspoon cheese. Broil 2 minutes or until cheese melts. Place rounds on a platter; drizzle evenly with the reduced poaching liquid. Serve warm.