Heat a large skillet over medium-high heat. Add oil; swirl to coat. Sprinkle pork with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add pork to pan; cook 10 minutes, turning to brown all sides. Transfer pork to a slow cooker. Add onion, bell pepper, and garlic to pan; sauté 5 minutes or until tender. Transfer mixture to slow cooker. Add remaining 1/4 teaspoon salt, 1/4 teaspoon pepper, tomatoes, rosemary, and thyme. Cover and cook on LOW for 7 1/2 hours. Add vinegar and Worcestershire sauce. Cook, uncovered, for 30 minutes; turn off heat. Shred pork with 2 forks.
Combine 2 cups water and milk in a saucepan over medium heat; bring to a boil. Reduce heat to low; add polenta, stirring with a whisk. Cook 4 minutes, stirring frequently. Remove from heat; stir in cheese and remaining 1/4 teaspoon pepper. Serve pork over polenta in shallow bowls.
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