Cut the bell pepper in half lengthwise, discarding seeds and membranes. Place the pepper halves, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil for 10 minutes or until blackened. Place in a zip-top plastic bag; seal. Let stand 10 minutes. Peel and finely chop.
Place the chopped green tomato, vinegar, sugar, and salt in a blender or food processor, and process until smooth.
Place the tomato mixture in a small saucepan; cook over medium-high heat 5 minutes or until liquid almost evaporates. Place in a small bowl; cool to room temperature. Add roasted pepper, onion, and garlic to tomato mixture; toss well.