1. Bring 3 cups water and 1/2 teaspoon salt to a boil in a medium saucepan. Gradually add polenta, stirring constantly with a whisk. Reduce heat to low, and cook 20 minutes, stirring frequently. Stir in 1 tablespoon oil and 1 tablespoon butter. Spoon polenta into an 8-inch square glass or ceramic baking dish coated with cooking spray. Press plastic wrap onto surface of polenta; chill 2 hours or until firm.
2. Heat a large skillet over medium-high heat. Add remaining 1 tablespoon oil and remaining 1 tablespoon butter; swirl to coat. Add garlic; sauté 15 seconds. Add remaining 1/4 teaspoon salt, onion, and thyme sprig; sauté 3 minutes or until onion begins to soften. Reduce heat to low; cook 30 minutes or until onion is very tender, stirring frequently. Add vinegar; cook 5 minutes, stirring frequently. Discard thyme sprig.
3. Invert polenta onto a cutting board; cut into 16 squares. Cut each square in half diagonally. Heat a skillet over medium heat. Lightly coat polenta triangles with cooking spray. Add 16 triangles to pan; cook 5 minutes on each side or until lightly browned. Repeat procedure with remaining triangles.
4. Divide onion mixture, bell pepper, and feta evenly among triangles. Garnish with thyme leaves.