Heat a medium nonstick skillet coated with cooking spray over medium-high heat. Add garlic; sauté 1 minute. Add spinach; cook 1 minute or until spinach wilts. Remove from heat.
Combine water and broth in a large saucepan over medium-high heat; bring to a boil. Gradually add polenta, stirring constantly with a whisk. Reduce heat to medium; cook 20 minutes, stirring frequently. Remove from heat; stir in spinach mixture, salt, and pepper.
Spoon polenta mixture evenly into a 9-inch springform pan coated with cooking spray. Press plastic wrap onto surface of polenta; chill 2 hours or until firm.
Preheat oven to 400°.
Remove polenta from pan; place on a baking sheet coated with cooking spray. Sprinkle the cheese evenly over polenta. Bake at 400° for 15 minutes or until cheese melts and begins to brown. Remove from oven; cool 5 minutes. Cut polenta into wedges, using a sharp knife. Serve immediately.