Heat oven to 400° F. Line 2 shallow roasting pans with parchment paper or foil. Arrange the tomatoes in one pan and the onion and garlic in the other. Drizzle both with the oil and vinegar, season with the salt and pepper, and toss to coat. Roast until the tomatoes split (20 minutes) and the onion and garlic have softened (25 minutes). Transfer the tomatoes, onion, garlic, and some of the juices to a bowl. Adjust seasoning, if desired. Add the herb and toss. Let cool. Cover and refrigerate for at least 3 hours. Serve cold or at room temperature.