To prepare pork, combine first 9 ingredients in a large zip-top plastic bag; seal and marinate in refrigerator overnight or up to 2 days.
Preheat oven to 400°.
Remove pork from bag. Place pork on a broiler pan coated with cooking spray. Bake at 400° for 30 minutes or until a thermometer registers 155°. Remove from oven; cover and let stand 10 minutes before slicing.
To prepare polenta, heat a medium saucepan over medium-high heat. Coat pan with cooking spray. Add onion and 2 garlic cloves; sauté 2 minutes. Add wine; cook 5 minutes or until liquid almost evaporates. Add 2 1/2 cups water; reduce heat, and simmer 5 minutes. Gradually add polenta, stirring constantly with a whisk. Cook over medium heat 15 minutes or until thick and creamy, stirring frequently and gradually adding remaining 2 1/2 cups water. Stir in butter, 1 teaspoon salt, and 1/4 teaspoon pepper. Serve with pork.