Preheat your pressure cooker in sauté mode.
Add the olive oil, chopped onion, and minced garlic. Sauté until soft and translucent.
Place the halved tomatoes in the pot, cut side up. Sprinkle with salt, pepper, and dried basil.
Pour in the vegetable broth and add balsamic vinegar if using. Close the lid and set the pressure to high for 10 minutes.
Once cooking time is up, use a quick release to vent the pressure. Carefully open the lid when safe to do so.
Use an immersion blender to purée the soup until smooth, or transfer to a blender in batches if necessary.
Taste and adjust seasoning before serving, adding more salt, pepper, or basil if needed.