1. Preheat your oven to 400°F (200°C).
2. On a large baking sheet, arrange the halved tomatoes, quartered onion, and garlic cloves.
3. Drizzle the olive oil over the vegetables and season with salt and black pepper.
4. Roast in the preheated oven for 30-40 minutes, or until the tomatoes are soft and slightly charred.
5. Remove from the oven and let cool slightly.
6. Transfer the roasted vegetables to a blender or food processor.
7. Add the balsamic vinegar, dried basil, dried oregano, and red pepper flakes (if using).
8. Blend until smooth, adjusting the seasoning as needed.
9. Pour the sauce into a saucepan and simmer over medium heat for 10-15 minutes to thicken and blend the flavors.
10. Serve immediately, or let cool and store in airtight containers in the refrigerator for up to a week, or freeze for longer storage.
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