Press the "Sautee" button on the pressure cooker to warm oil for a minute. Add onions and garlic and cook for about 5 minutes, stirring frequently, until the onions become appear translucent.
Press "Cancel" and add the tomatoes, vegetable broth, basil, salt, and pepper.
Close the lid, set the valve on "sealing", and cook on high pressure for 15 minutes. When finished, let the pressure release on its own for about 10 minutes before quick-releasing any remaining pressure. Carefully open the lid.
Stir in milk. Either using an immersion blender or using your blender, blend the soup until smooth and creamy. Adjust seasoning, if needed, and serve.
Store: Cool down the soup completely before storing in an air-tight container in the fridge for up to 4 days.
Freeze: The soup can be stored in a freezer-safe container for up to 3 months in the freezer. Thaw before reheating by leaving in the fridge the night before.
Reheat: Pour soup into a saucepan and warm on medium heat until the soup is hot.
Blending: If using a blender, be sure to divide and puree the soup in smaller batches to prevent overfilling the blender. I like to place a dish towel on top of the lid for extra protection.
Fresh tomatoes: If using fresh tomatoes, add an extra 1/2 cup or more, if needed, of vegetable broth.
Roasted tomatoes: For a deeper flavor and if you have additional time, roast the tomatoes in the oven before starting the soup.
Add carrots: For an extra serving of vegetables, add a cup of chopped carrots when adding the garlic and onion. They will add natural sweetness to the soup.